Since version 20 when I close a tab after some time tabs don't close and you can't even switch to the closed tab. Which seems to indicate that webpage has close but tab bar has not removed the page entry. This means that tab bar gets filled with old tabs quickly. Only way to recover is to close Firefox and restart. This is very annyoing and forcing me to switch to other browsers. Same problem doesn't happen on older version. Due to this I have right now downgraded to version 15. But our IT will automatically upgrade to latest version very soon.