Mozilla Backup is an excellent, excellent tool for making backups of your Mozilla, Firefox and Thunderbird profiles. Something that should be included in those applications in some similar form or incarnation.
That said, I just lost my latest Firefox profile from today's Windows XP clean-up-the-junk-by-reformatting operation, even after I remembered to backup my profile immediately before the reformat. Why? Well, not through any fault of Mozilla Backup. Not because Firefox was acting up. Two hints for you to make some intelligent guesswork, if you're so inclined. One, the Firefox profile folder has been changed in recent nightlies to "Firefox" instead of "Phoenix". Two, I was using Mozilla Backup 1.3a. OK probably a bonus hint is in order: Mozilla Backup 1.3a predates the change in the location of the profile folder. Dead giveaway, and I didn't see that one coming.
Mozilla Backup 1.3a backed up my profile in the "Phoenix" folder, which unfortunately has been untouched since my use of nightlies after 2003-03-12 - it was the dud profile that was left behind after being imported into the current "Firefox" profile folder. An entire month of bookmarks all lost in digital oblivion. Ouch.
At this point, I think it is prudent to state again that this is no fault of Mozilla Backup, more of a(n) (oxymoronic) reasonable folliness on my part. In fact, Pavel Cvrcek (the developer) has been keeping up with that profile folder change, as you'd be able to tell from recent Mozilla Backup nightlies. I love Mozilla Backup and I'll keep using it. Until the time comes, if it does come, that profiles can be imported/exported (or saved/restored) via a native browser UI.
Bugs 22689 and 187564 had previously been filed in 1999 and 2003 respectively requesting for the same functionality. Interesting to see the initial comments that users would be able to backup profiles by saving their profile directories (and subsequently copying back into a new profile when restoring). Not something very nice for non-technical users of course, considering how getting to the profile folder itself could pose some problems. Not pointing any fingers though, because there is a different time and context when some of those comments were made. It is good to see that someone has been assigned to bug 187564. Vote for those bugs if you think this is important to you too! Submit patches if you know how to help.