backing them up is easy:
In Firefox go to Bookmarks > Manage Bookmarks
Then in the Bookmark Manager that pops up:
File > Export....
Then it will export all of your bookmarks into an html that you can save anywhere you'd like. Save it to the desktop so you don't forget where it is. Importing them again is the same thing just instead of choosing "Export..." in the Bookmarks Manager choose "Import..." and load the file.