--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -4405,43 +4405,53 @@
             },
 
             clearTimer(timer) {
               timer.cancel();
             },
 
             getTabState(tab) {
               let state = this.tabState.get(tab);
+
+              // As an optimization, we lazily evaluate the state of tabs
+              // that we've never seen before. Once we've figured it out,
+              // we stash it in our state map.
               if (state === undefined) {
                 state = this.STATE_UNLOADED;
 
                 if (tab && tab.linkedPanel) {
                   let b = tab.linkedBrowser;
                   if (b.renderLayers && b.hasLayers) {
                     state = this.STATE_LOADED;
                   } else if (b.renderLayers && !b.hasLayers) {
                     state = this.STATE_LOADING;
                   } else if (!b.renderLayers && b.hasLayers) {
                     state = this.STATE_UNLOADING;
                   }
                 }
+
+                this.setTabStateNoAction(tab, state);
               }
 
               return state;
             },
 
             setTabStateNoAction(tab, state) {
               if (state == this.STATE_UNLOADED) {
                 this.tabState.delete(tab);
               } else {
                 this.tabState.set(tab, state);
               }
             },
 
             setTabState(tab, state) {
+              if (state == this.getTabState(tab)) {
+                return;
+              }
+
               this.setTabStateNoAction(tab, state);
 
               let browser = tab.linkedBrowser;
               let {tabParent} = browser.frameLoader;
               if (state == this.STATE_LOADING) {
                 this.assert(!this.minimizedOrFullyOccluded);
 
                 if (!this.tabbrowser.tabWarmingEnabled) {
@@ -4805,20 +4815,22 @@
                 this.loadingTab = null;
                 if (this.loadTimer) {
                   this.clearTimer(this.loadTimer);
                   this.loadTimer = null;
                 }
               }
 
               // If we're not loading anything, try loading the requested tab.
-              let requestedState = this.getTabState(this.requestedTab);
+              let stateOfRequestedTab = this.getTabState(this.requestedTab);
               if (!this.loadTimer && !this.minimizedOrFullyOccluded &&
-                  (requestedState == this.STATE_UNLOADED ||
-                   requestedState == this.STATE_UNLOADING)) {
+                  (stateOfRequestedTab == this.STATE_UNLOADED ||
+                   stateOfRequestedTab == this.STATE_UNLOADING ||
+                   this.warmingTabs.has(this.requestedTab))) {
+                this.assert(stateOfRequestedTab != this.STATE_LOADED);
                 this.loadRequestedTab();
               }
 
               // See how many tabs still have work to do.
               let numPending = 0;
               let numWarming = 0;
               for (let [tab, state] of this.tabState) {
                 // Skip print preview browsers since they shouldn't affect tab switching.
@@ -4927,16 +4939,17 @@
                 return;
               }
 
               this.logState(`onLayersReady(${tab._tPos}, ${browser.isRemoteBrowser})`);
 
               this.assert(this.getTabState(tab) == this.STATE_LOADING ||
                           this.getTabState(tab) == this.STATE_LOADED);
               this.setTabState(tab, this.STATE_LOADED);
+              this.unwarmTab(tab);
 
               if (this.loadingTab === tab) {
                 this.clearTimer(this.loadTimer);
                 this.loadTimer = null;
                 this.loadingTab = null;
               }
             },
 
@@ -5148,18 +5161,16 @@
                   }
                 } else if (this.canWarmTab(tab)) {
                   warmingState = "notWarmed";
                 }
 
                 Services.telemetry
                         .getHistogramById("FX_TAB_SWITCH_REQUEST_TAB_WARMING_STATE")
                         .add(warmingState);
-
-                this.unwarmTab(tab);
               }
 
               this._requestingTab = true;
               this.logState("requestTab " + this.tinfo(tab));
               this.startTabSwitch();
 
               this.requestedTab = tab;