Firefox has a convenient function in its Bookmark Manager (Bookmarks / Bookmark Manager). Once you've opened BookMark Manager, which opens in a new window, just click on File, Export and choose where to save the bookmarks file. By default, the file titled bookmarks.html.
Of course, if you ever need to restore the file — or to import it into a new Firefox installation — there is a similar Import function there.
That makes it easy to back up the Firefox bookmarks. It also makes it very easy to take a copy of them with you -- on a floppy disk, a CDROM or a USB Flash Drive, or on any type of memory card (Compact Flash, Memory Stick, SD/MMC card, etc) if you have a card reader to take along.