The Mozilla website has a whole page on restoring your bookmarks here

Also, after you have them restored, I would turn on syncronization.

It’s great. When I start using a new computer, I turn on sync and BAM! – I have all my bookmarks.

And it continues to sync – changes on one computer are copied to the others.

I can heartily recommend Xmarks”:https://www.xmarks.com/ for bookmark syncing. It works across browsers, so you can have the same bookmarks in Firefox, IE, Safari and Chrome.

Step 1: Click the Windows Start button and type %APPDATA%\Mozilla\Firefox\Profiles\ in the Search box at the bottom of the Start menu, without pressing Enter. A list of profiles will appear at the top of the Start menu. Click on the profile with “default” in the name to open it in a window. same resource as @Crazydawg pointed out.
Step 2: Safely put away the lastest .json file from the bookmarkbackups folder. Reinstall Firefox and go to
Bookmarks > Organize Bookmarks > Import & Backup > Restore > Choose File (.json)
Done!

@jaytkay Firefox has Sync built in. Open the menu (hamburger icon at the right-hand side) and click “Sign in to Sync” at the bottom. Very useful.

@Leonidas_300 Do keep in mind that Windows XP is no longer supported, and hence potential security holes will not get closed. If you want to do online banking or something, it’s really not a good idea to do that on a computer with Windows XP. As an alternative, you could buy a newer version of Windows, or install something free like Ubuntu.