They are saved in your Users\Username\AppData\Roaming\Mozilla\Profiles\profilename\ folder and are automatically backed up in the bookmarkbackups folder under the date they were backed up (last 10 copies saved by default) as a .json file
To find the folder that your Firefox bookmarks or in 1. Click on "Help" Menu. 2. Click on "Troubleshooting Information". 3. Click on "Open Containing Folder". This is the default folder that Firefox stores your bookmarks in.