Quite good and useful Rated 4 out of 5 stars
This is a very useful add-on, and works quite well.
The only things I found to grumble about were:
1) The backup is sometimes incomplete (I usually backup just before closing a session - and there is no safeguard against a session closing before the backup is finished saving - so you have to wait and guess when it is done).
2) Whenever you install FEBE it it loads a welcome page (some people might be fine with this - but if you use it often the page becomes annoying).
3) There is no way of backing up to an installable xpi (yes, someone else has mentioned this - I mention it again because it would be VERY useful - just drag the xpi to a fresh Firefox install, and you're done - no first installing FEBE, then restarting, then creating a new profile, restoring to that profile, and then restarting using that profile).
But despite those (perhaps minor) complaints this is, at the moment, the best Firefox backup extension available.