There are two easy ways to save the backups,
Click on bookmarks then organize then backup ,save to desktop then email to yourself as attachment,
or click bookmarks then organize then export as html then save it to desktop and email to yourself as attachment .
After upgrade import the saved file back into firefox .
If you want to save the whole profile its just as easy.
Navigate to C:\Users\username\AppData\Roaming\Mozilla\Firefox\Profiles\xxxxxxxx.default
username is your logon name the xx's are the random letters FF names the default folder
select all files and folders in profile and using 7zip right click and select add to archive
then again email file to your self.
then after you can unzip the file and copy and replace the profile files into your new installation.