Getting Started
Now that you have already gotten firefox installed let take steps to backup your bookmarks.
The Process
This is a pretty simple backup procedure. First open Firefox. From there do as you would to import. But select export.
Go under Bookmarks and select Organize. From there click on the Import and Backup feature. From there select the export HTML option.

Now the Wizard will come up just like it did for the Import Bookmark. Now here it will give you the option to save them right to your desktop. Now while this is a good option and you can save it there I would suggest against it. If you end up having a system failure and have to format and reinstall the file woudl be lost. I would suggest placing the file someplace where you can access it after a critical system failure.
Now here just navigate the Wizard to this location and then just click Save.

Finishing Up
That's it. Backup saved to a location for future use. If you go thru bookmarks often I would suggest you backup often for safe keeping.