Tabs remain renamed after restarting Firefox, that's what I expected the most!
Little problem : the tab which is active while closing the program loses it's name when restart. The other ones keep their name. So I always close Firefox after opening a random tab, but it would be good to fix this bug.
Could be improved : a shortcut to open the renaming dialog box
Could be great : a functionnality to automatically rename a tab using a specific part of a web link, or a specific text (always at the same place) of a page.