--- a/browser/components/sessionstore/test/browser_sessionStorage_size.js
+++ b/browser/components/sessionstore/test/browser_sessionStorage_size.js
@@ -5,34 +5,46 @@
 
 const RAND = Math.random();
 const URL = "http://mochi.test:8888/browser/" +
             "browser/components/sessionstore/test/browser_sessionStorage.html" +
             "?" + RAND;
 
 const OUTER_VALUE = "outer-value-" + RAND;
 
+function getEstimateChars() {
+  let snap;
+  if (gMultiProcessBrowser) {
+    snap = Services.telemetry.histogramSnapshots.content["FX_SESSION_RESTORE_DOM_STORAGE_SIZE_ESTIMATE_CHARS"];
+  } else {
+    snap = Services.telemetry.histogramSnapshots.parent["FX_SESSION_RESTORE_DOM_STORAGE_SIZE_ESTIMATE_CHARS"];
+  }
+  if (!snap) {
+    return 0;
+  }
+  return snap.counts[4];
+}
+
 // Test that we record the size of messages.
 add_task(async function test_telemetry() {
   Services.telemetry.canRecordExtended = true;
-  let suffix = gMultiProcessBrowser ? "#content" : "";
-  let histogram = Services.telemetry.getHistogramById("FX_SESSION_RESTORE_DOM_STORAGE_SIZE_ESTIMATE_CHARS" + suffix);
-  let snap1 = histogram.snapshot();
+
+  let prev = getEstimateChars()
 
   let tab = BrowserTestUtils.addTab(gBrowser, URL);
   let browser = tab.linkedBrowser;
   await promiseBrowserLoaded(browser);
 
   // Flush to make sure we submitted telemetry data.
   await TabStateFlusher.flush(browser);
 
   // There is no good way to make sure that the parent received the histogram entries from the child processes.
   // Let's stick to the ugly, spinning the event loop until we have a good approach (Bug 1357509).
   await BrowserTestUtils.waitForCondition(() => {
-    return histogram.snapshot().counts[4] > snap1.counts[4];
+    return getEstimateChars() > prev;
   });
 
   Assert.ok(true);
   await promiseRemoveTab(tab);
   Services.telemetry.canRecordExtended = false;
 });
 
 // Lower the size limit for DOM Storage content. Check that DOM Storage

--- a/dom/base/test/browser_use_counters.js
+++ b/dom/base/test/browser_use_counters.js
@@ -103,25 +103,35 @@ function waitForPageLoad(browser) {
       }
       addEventListener("load", listener, true);
     });
   });
 }
 
 function grabHistogramsFromContent(use_counter_middlefix, page_before = null) {
   let telemetry = Cc["@mozilla.org/base/telemetry;1"].getService(Ci.nsITelemetry);
-  let suffix = Services.appinfo.browserTabsRemoteAutostart ? "#content" : "";
-  let gather = () => [
-    telemetry.getHistogramById("USE_COUNTER2_" + use_counter_middlefix + "_PAGE" + suffix).snapshot().sum,
-    telemetry.getHistogramById("USE_COUNTER2_" + use_counter_middlefix + "_DOCUMENT" + suffix).snapshot().sum,
-    telemetry.getHistogramById("CONTENT_DOCUMENTS_DESTROYED" + suffix).snapshot().sum,
-    telemetry.getHistogramById("TOP_LEVEL_CONTENT_DOCUMENTS_DESTROYED" + suffix).snapshot().sum,
-  ];
+  let gather = () => {
+    let snapshots;
+    if (Services.appinfo.browserTabsRemoteAutostart) {
+      snapshots = telemetry.histogramSnapshots.content;
+    } else {
+      snapshots = telemetry.histogramSnapshots.parent;
+    }
+    let checkGet = (probe) => {
+      return snapshots[probe] ? snapshots[probe].sum : 0;
+    };
+    return [
+      checkGet("USE_COUNTER2_" + use_counter_middlefix + "_PAGE"),
+      checkGet("USE_COUNTER2_" + use_counter_middlefix + "_DOCUMENT"),
+      checkGet("CONTENT_DOCUMENTS_DESTROYED"),
+      checkGet("TOP_LEVEL_CONTENT_DOCUMENTS_DESTROYED"),
+    ];
+  };
   return BrowserTestUtils.waitForCondition(() => {
-    return page_before != telemetry.getHistogramById("USE_COUNTER2_" + use_counter_middlefix + "_PAGE" + suffix).snapshot().sum;
+    return page_before != gather()[0];
   }).then(gather, gather);
 }
 
 var check_use_counter_iframe = async function(file, use_counter_middlefix, check_documents=true) {
   info("checking " + file + " with histogram " + use_counter_middlefix);
 
   let newTab = BrowserTestUtils.addTab(gBrowser,  "about:blank");
   gBrowser.selectedTab = newTab;

--- a/dom/ipc/tests/browser_remote_navigation_delay_telemetry.js
+++ b/dom/ipc/tests/browser_remote_navigation_delay_telemetry.js
@@ -8,45 +8,42 @@ add_task(async function test_memory_dist
     return;
   }
 
   await SpecialPowers.pushPrefEnv({set: [["toolkit.telemetry.enabled", true]]});
   let canRecordExtended = Services.telemetry.canRecordExtended;
   Services.telemetry.canRecordExtended = true;
   registerCleanupFunction(() => Services.telemetry.canRecordExtended = canRecordExtended);
 
-  // Note the #content suffix after the id. This is the only way this API lets us fetch the
-  // histogram entries reported by a content process.
-  let histogram = Services.telemetry.getKeyedHistogramById("FX_TAB_REMOTE_NAVIGATION_DELAY_MS#content");
-  histogram.clear();
+  Services.telemetry.snapshotSubsessionKeyedHistograms(true /*clear*/);
 
   // Open a remote page in a new tab to trigger the WebNavigation:LoadURI.
   let tab1 = await BrowserTestUtils.openNewForegroundTab(gBrowser, "http://example.com");
   ok(tab1.linkedBrowser.isRemoteBrowser, "|tab1| should have a remote browser.");
 
   // Open a new tab with about:robots, so it ends up in the parent process with a non-remote browser.
   let tab2 = await BrowserTestUtils.openNewForegroundTab(gBrowser, "about:robots");
   ok(!tab2.linkedBrowser.isRemoteBrowser, "|tab2| should have a non-remote browser.");
   // Navigate the tab, so it will change remotness and it triggers the SessionStore:restoreTabContent case.
   await BrowserTestUtils.loadURI(tab2.linkedBrowser, "http://example.com");
   ok(tab2.linkedBrowser.isRemoteBrowser, "|tab2| should have a remote browser by now.");
 
   // There is no good way to make sure that the parent received the histogram entries from the child processes.
   // Let's stick to the ugly, spinning the event loop until we have a good approach (Bug 1357509).
   await BrowserTestUtils.waitForCondition(() => {
-    let s = histogram.snapshot();
-    return "WebNavigation:LoadURI" in s && "SessionStore:restoreTabContent" in s;
+    let s = Services.telemetry.snapshotSubsessionKeyedHistograms().content["FX_TAB_REMOTE_NAVIGATION_DELAY_MS"];
+    return s && "WebNavigation:LoadURI" in s && "SessionStore:restoreTabContent" in s;
   });
 
-  let s = histogram.snapshot();
+  let s = Services.telemetry.snapshotSubsessionKeyedHistograms().content["FX_TAB_REMOTE_NAVIGATION_DELAY_MS"];
   let restoreTabSnapshot = s["SessionStore:restoreTabContent"];
   ok(restoreTabSnapshot.sum > 0, "Zero delay for the restoreTabContent case is unlikely.");
   ok(restoreTabSnapshot.sum < 10000, "More than 10 seconds delay for the restoreTabContent case is unlikely.");
 
   let loadURISnapshot = s["WebNavigation:LoadURI"];
   ok(loadURISnapshot.sum > 0, "Zero delay for the LoadURI case is unlikely.");
   ok(loadURISnapshot.sum < 10000, "More than 10 seconds delay for the LoadURI case is unlikely.");
 
-  histogram.clear();
+  Services.telemetry.snapshotSubsessionKeyedHistograms(true /*clear*/);
 
   await BrowserTestUtils.removeTab(tab2);
   await BrowserTestUtils.removeTab(tab1);
 });

--- a/toolkit/components/osfile/tests/xpcshell/test_telemetry.js
+++ b/toolkit/components/osfile/tests/xpcshell/test_telemetry.js
@@ -22,42 +22,42 @@ function getCount(histogram) {
 }
 
 // Ensure that launching the OS.File worker adds data to the relevant
 // histograms
 add_task(async function test_startup() {
   let LAUNCH = "OSFILE_WORKER_LAUNCH_MS";
   let READY = "OSFILE_WORKER_READY_MS";
 
-  let before = Services.telemetry.histogramSnapshots;
+  let before = Services.telemetry.histogramSnapshots.parent;
 
   // Launch the OS.File worker
   await File.getCurrentDirectory();
 
-  let after = Services.telemetry.histogramSnapshots;
+  let after = Services.telemetry.histogramSnapshots.parent;
 
 
   do_print("Ensuring that we have recorded measures for histograms");
   do_check_eq(getCount(after[LAUNCH]), getCount(before[LAUNCH]) + 1);
   do_check_eq(getCount(after[READY]), getCount(before[READY]) + 1);
 
   do_print("Ensuring that launh <= ready");
   do_check_true(after[LAUNCH].sum <= after[READY].sum);
 });
 
 // Ensure that calling writeAtomic adds data to the relevant histograms
 add_task(async function test_writeAtomic() {
   let LABEL = "OSFILE_WRITEATOMIC_JANK_MS";
 
-  let before = Services.telemetry.histogramSnapshots;
+  let before = Services.telemetry.histogramSnapshots.parent;
 
   // Perform a write.
   let path = Path.join(Constants.Path.profileDir, "test_osfile_telemetry.tmp");
   await File.writeAtomic(path, LABEL, { tmpPath: path + ".tmp" } );
 
-  let after = Services.telemetry.histogramSnapshots;
+  let after = Services.telemetry.histogramSnapshots.parent;
 
   do_check_eq(getCount(after[LABEL]), getCount(before[LABEL]) + 1);
 });
 
 function run_test() {
   run_next_test();
 }

--- a/toolkit/components/terminator/tests/xpcshell/test_terminator_reload.js
+++ b/toolkit/components/terminator/tests/xpcshell/test_terminator_reload.js
@@ -28,17 +28,17 @@ var HISTOGRAMS = {
 add_task(async function init() {
   do_get_profile();
   PATH = Path.join(Constants.Path.localProfileDir, "ShutdownDuration.json");
 });
 
 add_task(async function test_reload() {
   do_print("Forging data");
   let data = {};
-  let telemetrySnapshots = Services.telemetry.histogramSnapshots;
+  let telemetrySnapshots = Services.telemetry.histogramSnapshots.parent;
   let i = 0;
   for (let k of Object.keys(HISTOGRAMS)) {
     let id = HISTOGRAMS[k];
     data[k] = i++;
     Assert.equal(telemetrySnapshots[id] || undefined, undefined, "Histogram " + id + " is empty");
   }
 
 
@@ -59,17 +59,17 @@ add_task(async function test_reload() {
   let tt = Cc["@mozilla.org/toolkit/shutdown-terminator-telemetry;1"].
     createInstance(Ci.nsIObserver);
   tt.observe(null, "profile-after-change", "");
 
   do_print("Waiting until telemetry is updated");
   // Now wait until Telemetry is updated
   await wait;
 
-  telemetrySnapshots = Services.telemetry.histogramSnapshots;
+  telemetrySnapshots = Services.telemetry.histogramSnapshots.parent;
   for (let k of Object.keys(HISTOGRAMS)) {
     let id = HISTOGRAMS[k];
     do_print("Testing histogram " + id);
     let snapshot = telemetrySnapshots[id];
     let count = 0;
     for (let x of snapshot.counts) {
       count += x;
     }