Firefox creates backups of your bookmarks in a folder on your computer. If you want to check its contents, it's named bookmarkbackups and stored in your currently active Firefox profile folder. The backups are compressed, i.e., unreadable, but I have a tool to convert them to a web page if you need something readable or want to confirm what's in them.

• Profiles - Where Firefox stores your bookmarks, passwords and other user data
• My converter: https://www.jeffersonscher.com/ffu/bookbackreader.html

Note that Sync isn't designed to be used as a backup and that you should backup your personal data yourself locally.

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

You can also create an HTML bookmarks backup.

• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer
• https://support.mozilla.org/en-US/kb/import-bookmarks-html-file

So let's say I have one PC with firefox sync....that PC stops working. I get a new PC and login to my firefox account.

Will that new PC have bookmarks?