Sure there are Firefox extensions out there that let you save your Firefox preference tweaks, but if you like the security of knowing you've done a manual backup, the MakeUseOf blog explains how to do it. For Windows users, you'll need to go to your Firefox profile folder in Windows Explorer. The default location is Documents and Settings > Mozilla > Firefox > Profiles.
The file you're looking for is prefs.js. Copy it to a USB stick or burn it to CD.
If you need to reinstall Firefox, drag your backup copy of prefs.js into your Firefox profile. You'll get a message asking if you want to replace the existing prefs.js file with your copied file -say yes, and you'll have restored your Firefox preferences the way you wanted them.
Of course there's no point backing up if you don't do it regularly - so be sure to update your saved prefs.js file when you tweak your Firefox settings.
Quick tip: Backup Firefox preferences [MakeUseOf.com]