You can create backups in two formats:

• JSON format: Restore bookmarks from backup or move them to another computer (manual backup section)
• HTML format: Export Firefox bookmarks to an HTML file to back up or transfer bookmarks

That said, normally there's no problem, Firefox upconverts the files during the upgrade. However, your Firefox identified itself as version 66, which is very old now. You might want to upgrade in steps. For example, install version 72, then 78, then finally 88.

The following article has a link to all the old installers: Install an older version of Firefox

Please follow the complete steps in the following article, and run Firefox twice after each update (run, quit, run) and keep it running at least 5 minutes before the next update so that any deferred actions which occur shortly after startup have time to be completed.

How to download and install Firefox on Mac

Note: the JSON files are coded while the HTML files are plain text. Sometimes the JSON file gets corrupted.

Only an automatically created .jsonlz4 backup in the bookmarkbackups folder not readable as these files are compressed, a manually created JSON backup is still a plain text file, but is harder to read because of the JSON format ({key:value} data).