If you keep a slew of tabs open in Firefox, you've probably used the "List All Tabs" menu on more than one occasion. With a few about:config tweaks, you can add thumbnails to this menu for even simpler tab jumping.
Having a ton of tabs open makes navigating between them difficult, and the List All Tabs menu is helpful, but not perfect. If you have too many tabs to sort through on the tab bar, it's probably just as hard to sort through them in a text-only list. Firefox has had a hidden feature since 3.6, though, that adds thumbnails to this list for quick and easy tab switching. To enable it, head to about:config in your address bar, promise Firefox you'll be careful, and search for browser.ctrlTab.previews. Double-click on it to set it to True. Do the same thing for browser.allTabs.previews and you should notice the List All Tabs button has a different icon. Click on it and you'll get something like the image above, with an overview of all your tabs, thumbnails in tow.
This feature's been around for awhile, but has yet to actually be integrated into Firefox. It seems to work more than fine in my tests, so there shouldn't be any harm in trying it out. It's also a good deal faster than Firefox 4's "Tab Groups" feature, which performs a similar (but not identical) function.
Tab Preview Revisited [Firefox Extension Guru's Blog]