Hi Dave!
All of your Firefox data is saved in a separate folder, which is usually something like:
C:\Documents and Settings\[yourname]\Application Data\Mozilla\Firefox\Profiles\[xxxxxxx] (it may be different on yours, as I also have Mozilla, but up to Application Data will be the same)
In there you will find Bookmarks.html, which you can save and restore, by simply copying and pasting. Of course, you could just save the entire profile folder, but if you format, make sure the profile is back in place for when you reinstall Firefox, or it will create a new profile. If that happens, it's easy enough to transfer the data from the old saved profile, by copying and pasting. Other useful files to save in the profile folder are:
prefs.js (stores all your settings)
signons.txt (stored passwords)
For Thunderbird, there is a separate profile folder, also in the same Application Data folder, which contains all your mail accounts in the Mail folder. To back up, simply copy the folder, and also the prefs.js, to restore the settings. You will also find abook.mab in that folder, which is your address book. Again, after a format, make sure the profile is back in place before reinstalling Thunderbird, if you intend to use your old profile.
Personally, I would recommend creating new profiles when you format, and just import the backed up files you need, by overwriting the new ones. If you use a complete old profile, it may carry with it any errors which were present before the format.
Hope that helps!
P.S. Just come across
this, which might also be of interest, but I haven't used it myself, as I prefer to back up manually.