Firefox keeps your bookmarks in a file called bookmarks.html in your profile folder.

It also automatically backs up your bookmarks to a folder called bookmarkbackups, also in your profile folder.
If you want to back them up to another media simply copy the bookmarks.html file over.

Where is my profile folder

There are also addons for backing up and syncing.
https://addons.mozilla.org/en-US/fir.../type:1/cat:22