--- a/browser/components/preferences/in-content/advanced.js
+++ b/browser/components/preferences/in-content/advanced.js
@@ -223,17 +223,17 @@ var gAdvancedPane = {
     return checkbox.checked ? 1 : 0;
   },
 
   /**
    * When the user toggles the layers.acceleration.disabled pref,
    * sync its new value to the gfx.direct2d.disabled pref too.
    */
   updateHardwareAcceleration() {
-    if (AppConstants.platform = "win") {
+    if (AppConstants.platform == "win") {
       var fromPref = document.getElementById("layers.acceleration.disabled");
       var toPref = document.getElementById("gfx.direct2d.disabled");
       toPref.value = fromPref.value;
     }
   },
 
   // DATA CHOICES TAB
 

--- a/toolkit/.eslintrc.js
+++ b/toolkit/.eslintrc.js
@@ -55,16 +55,19 @@ module.exports = {
     "linebreak-style": ["error", "unix"],
 
     // Always require parenthesis for new calls
     // "new-parens": "error",
 
     // Use [] instead of Array()
     // "no-array-constructor": "error",
 
+    // Disallow assignment operators in conditional statements
+    "no-cond-assign": "error",
+
     // Disallow the use of debugger
     "no-debugger": "error",
 
     // Disallow deleting variables
     "no-delete-var": "error",
 
     // No duplicate arguments in function declarations
     "no-dupe-args": "error",

--- a/toolkit/components/aboutperformance/content/aboutPerformance.js
+++ b/toolkit/components/aboutperformance/content/aboutPerformance.js
@@ -1011,17 +1011,17 @@ var SubprocessMonitor = {
           SubprocessMonitor.queueUpdate();
           return;
         }
         let resultTable = document.getElementById("subprocess-reports");
         let recycle = [];
         // We first iterate the table to check if summaries exist for rowPids,
         // if yes, update them and delete the pid's summary or else hide the row
         // for recycling it. Start at row 1 instead of 0 (to skip the header row).
-        for (let i = 1, row; row = resultTable.rows[i]; i++) {
+        for (let i = 1, row; (row = resultTable.rows[i]); i++) {
           let rowPid = row.dataset.pid;
           let summary = summaries[rowPid];
           if (summary) {
             // Now we update the values in the row, which is hardcoded for now,
             // but we might want to make this more adaptable in the future.
             SubprocessMonitor.updateRow(row, summaries, rowPid);
             delete summaries[rowPid];
           } else {

--- a/toolkit/components/contentprefs/tests/unit_cps2/head.js
+++ b/toolkit/components/contentprefs/tests/unit_cps2/head.js
@@ -336,17 +336,17 @@ function dbOK(expectedRows) {
 
   db.executeAsync([stmt], 1, {
     handleCompletion(reason) {
       arraysOfArraysOK(actualRows, expectedRows);
       next();
     },
     handleResult(results) {
       let row = null;
-      while (row = results.getNextRow()) {
+      while ((row = results.getNextRow())) {
         actualRows.push(cols.map(c => row.getResultByName(c)));
       }
     },
     handleError(err) {
       do_throw(err);
     }
   });
   stmt.finalize();

--- a/toolkit/components/places/ClusterLib.js
+++ b/toolkit/components/places/ClusterLib.js
@@ -79,18 +79,17 @@ HierarchicalClustering.prototype = {
 
         if (dist < distances[i][neighbors[i]]) {
           neighbors[i] = j;
         }
       }
     }
 
     // merge the next two closest clusters until none of them are close enough
-    let next = null, i = 0;
-    for (; next = this.closestClusters(clusters, distances, neighbors); i++) {
+    for (let next = null, i = 0; (next = this.closestClusters(clusters, distances, neighbors)); i++) {
       if (snapshotCallback && (i % snapshotGap) == 0) {
         snapshotCallback(clusters);
       }
       this.mergeClusters(clusters, distances, neighbors, clustersByKey,
                          clustersByKey[next[0]], clustersByKey[next[1]]);
     }
     return clusters;
   },

--- a/toolkit/content/aboutSupport.js
+++ b/toolkit/content/aboutSupport.js
@@ -452,18 +452,18 @@ var snapshotFormatters = {
         let trs = [];
         for (let entry of feature.log) {
           if (entry.type == "default" && entry.status == "available")
             continue;
 
           let contents;
           if (entry.message.length > 0 && entry.message[0] == "#") {
             // This is a failure ID. See nsIGfxInfo.idl.
-            let m;
-            if (m = /#BLOCKLIST_FEATURE_FAILURE_BUG_(\d+)/.exec(entry.message)) {
+            let m = /#BLOCKLIST_FEATURE_FAILURE_BUG_(\d+)/.exec(entry.message);
+            if (m) {
               let bugSpan = $.new("span");
               bugSpan.textContent = strings.GetStringFromName("blocklistedBug") + "; ";
 
               let bugHref = $.new("a");
               bugHref.href = "https://bugzilla.mozilla.org/show_bug.cgi?id=" + m[1];
               bugHref.textContent = strings.formatStringFromName("bugLink", [m[1]], 1);
 
               contents = [bugSpan, bugHref];

--- a/toolkit/mozapps/extensions/test/xpcshell/test_plugins.js
+++ b/toolkit/mozapps/extensions/test/xpcshell/test_plugins.js
@@ -67,17 +67,17 @@ function get_test_plugin() {
 
 function getFileSize(aFile) {
   if (!aFile.isDirectory())
     return aFile.fileSize;
 
   let size = 0;
   let entries = aFile.directoryEntries.QueryInterface(AM_Ci.nsIDirectoryEnumerator);
   let entry;
-  while (entry = entries.nextFile)
+  while ((entry = entries.nextFile))
     size += getFileSize(entry);
   entries.close();
   return size;
 }
 
 function getPluginLastModifiedTime(aPluginFile) {
   // On OS X we use the bundle contents last modified time as using
   // the package directories modified date may be outdated.