Firefox Devs Working on Tab Previews
In current versions of Firefox you hover your mouse over a non-active tab (i.e. any tab other than the one you’re currently viewing) to see (after a small delay) a tooltip containing the web page title.
Handy, especially if you have a ton of open tabs, including several from the same site, and aren’t able to see enough of the page title in the tab itself to know which page is where.
But in the latest Firefox beta builds Mozilla developers are experimenting with a major enhancement in the form of tab previews.