Deja vu all over again: tab previews are back in Firefox 3.6
Originally slated for 3.0, then bumped to 3.5, then bumped again, tab previews are back in Firefox 3.6. To activate the feature, you'll need to jump into your about:config and search for browser.ctrlTab.previews. Double click the entry to set its value to true, and the result is what you see above.
Your last six tabs are displayed in a floating window, with a button below that allows viewing of all open tabs (screenshot after the break).
Again, you'll need to open about:config and change a value - browser.ctrlTab.mostRecentlyUsed - and set it to true.