Your bookmarks should still be present after you update Firefox but here's how to back them up:
Go to Start > Search and run a search for 'bookmarks'. This will bring up several files. Some may have dates or numbers as part of the file names but you should find that two of them are simply named 'bookmarks.html'. One of these will be a very small file (probably about 7Kb). The other will be much bigger. This is your current bookmarks file. (Mine is 417Kb). Copy this file to a CD (or, as long as it's under 1.4Mb, to a floppy disk).
If you find that your updated version of Firefox doesn't have your old bookmarks, repeat the search for bookmarks.html and locate the relevant file, as before. This time, right-click on the file and select 'open containing folder'. If the window opens full-size, make it smaller. Now insert your CD (or floppy) and double-click on the icon for your CD (or floppy) drive. Again, ensure that the window only takes up part of the screen. Drag the bookmarks.html file from the CD/floppy window into the other open window. When you're asked to confirm that you wish to over-write the existing file, select 'Yes'.
I've had quite a few problems with computer crashes over the last few months. This has meant that I've ended up re-formatting the drives of two computers (and re-installing all of the software) several times. However, I've never lost my Firefox bookmarks because I've always used this back-up procedure to transfer my old bookmarks to my new installation of Firefox. So I know the method works ;-)
Chris