Taking a back up of bookmarks is pretty simple process
Go to Bookmarks->Organize Bookmarks in the main menu.
This will open up the Bookmarks Manager window. In this window you can see all the bookmarks saved.
Go to File->Export and save the file in the preferred location.
The file will contain all your bookmarks and will be an .html file.
Whenever you install Firefox in another PC or re-install in your PC, you can import all the bookmarks through File->Import and select the option From File in the Bookmarks Manager.
Similarly even in chrome , safari , IE the method in nearly same ...