In FF 31.0 on Win7/64, I've just started seeing this problem in recent days: When I click on a nearby tab, FF will sometimes instead show the contents of a MUCH EARLIER tab (I have a fair few open, more than 50). I think this also occasionlly happens when I delete a tab, which would be less surprising, but it certainly should not happen that I see tab Y when I click on tab X. My FF installation is pretty standard except that I've changed the hidden setting to put my tab bar below the URL bar and favourites, where it is supposed to be. The implementation is as ugly as sin in current FF versions, but at least it's still possible. Default theme for 31.0 Addons (all current and none updated recently): Adblock Plus Evernote Web Clipper FiddlerHook Flashblock Heartbleed Notifier Lazarus Form Recovery LightBeam Live HTTP Headers Noscript Open Bookmarks in New Tab YouTube High Def Download Statusbar (disabled)