We've written before about Foxmarks, which recently went through a name change to Xmarks. Though it now works with several browsers, it was popularized as a Firefox extension used by many mobile users who want to synchronize their bookmarks and profiles across devices. Good as it is, I also like to use MozBackup--another Firefox extension--for backing up profiles and many similar tasks. MozBackup is compatible with a slew of browsers, and lets you back up and restore bookmarks, mail, contacts, history, extensions, cache and more. Here's what you get with it.
You can use MozBackup to take backups of multiple profiles, and synchronize them on various portable devices, or carry them on a USB thumb drive. You can also use it to backup passwords, certificates and other security-oriented information. It will also backup and restore your Firefox extensions, which can be very handy if you use a lot of them, and you can password protect everything that MozBackup stores. To back up a profile, you just select one and supply the destination directory you want to save it to:
 
MozBackup is compatible with the following browsers and applications (note that you can backup Songbird music files with it, Thunderbird e-mail profiles, and more):
Firefox 1.0 - 3.1
Thunderbird 1.0 - 3.0 Beta 2
Sunbird 0.3 - 0.9
Flock 1.0 - 2.0
SeaMonkey 1.0a - 2.0 Alpha 3
Mozilla Suite 1.7 - 1.7.x
Spicebird 0.4 - 0.8
Songbird 1.0
Netscape 7.x, 9.x
 
Especially if you use many instances of Firefox across many kinds of devices, I recommend regularly backing up your profiles and extensions to a USB thumb drive with MozBackup. From there, you've got a quick way to get any instance of Firefox to work the way you want it to.
For many more useful Firefox extensions, see our Superguide.