Firefox bookmarks need backup when the web browser update,or you want to import bookmarks into another web browser.Export bookmarks to an HTML file is an easy way to backup.
First,at the top of firefox window,click to Bookmarks->Oragnize Bookmarks…

Then,go to Import and Backup–>Export HTML… in following screen

bookmark

Select a spotĀ  to save the backup file and click save.

bookmark1

Now you can import the bookmarks.html into the fresh firefox,or another web browser.