There is a lot of info online, but it all seems to be out of date!
If you are trying to Sync your bookmarks between Firefox on two separate computers, you can use Firefox Sync.
What is Firefox Sync?
How to sync Firefox settings between computers
If you want to sync between your desktop and mobile device, please see:
How do I sync Firefox between my desktop and mobile?
In order to use Firefox Sync, you will have to update to at least Firefox 4. Download Firefox