I have talked about it before, but it is worth mentioning again. MozBackup is simply the best backup utilitiy for Firefox. It is the only one I have been able to get to backup everything as well as restore. Most tools either do one or the other really well – MozBackup does both with no issues at all.
Over at Firefoxrawx, they have posted a really nice tutorial on how to backup using MozBackup. It is worth looking at if backing up is something that has always scared you.