Please view this knowledge base article: Restore bookmarks from backup or move them to another computer

Or you can use an add-on like febe, it can backup bookmarks, preferences and the mos important thing Extensions. The only problem is that requires some time like 5-15 minutes, depends of how much do you will back up Link to febe: https://addons.mozilla.org/en-US/firefox/addon/2109/

Weather and Jorge, thanks for your replies. I have saved the bookmarks now using the information on the knowledge base, but when I tried to open the file, it was a JSON file which Windows doesn't open. There were a number of links to sites to download something to open such a file, but I dont know which ones are safe and effective. Can you help any further with this?

When you get Firefox re-installed, open Bookmarks > Organize Bookmarks -> Restore - Choose File...