On Windows, have the bookmarks you see on Mac integrated into the same places (Bookmarks Toolbar, Bookmarks Menu, Other Bookmarks)? In other words, they aren't segregated into a category of Sync'd bookmarks like Mobile?

You might want to make a manual backup for safety and see how that works out. You probably know the steps, but just in case: Restore bookmarks from backup or move them to another computer.

If you open that .json file in a tab, Firefox will present a structured view of the file contents so you can see what is under each top-level category as you click to expand it. Does Firefox make a correct manual backup?

Are you closing Firefox on the Mac via Exit because a new backup is only created when you start Firefox and not when you keep Firefox running.

On Mac you can close all open windows, but keep Firefox running with the menu bar visible or minimized in the Doc.

Make sure to use "Quit Firefox" with all windows still open, especially if you want to use "Restore Previous Session".

See also the services.sync.engine.bookmarks.validation prefs in about:config.

On Windows, have the bookmarks you see on Mac integrated into the same places (Bookmarks Toolbar, Bookmarks Menu, Other Bookmarks)? In other words, they aren't segregated into a category of Sync'd bookmarks like Mobile? You might want to make a manual backup for safety and see how that works out. You probably know the steps, but just in case: Restore bookmarks from backup or move them to another computer. If you open that .json file in a tab, Firefox will present a structured view of the file contents so you can see what is under each top-level category as you click to expand it. Does Firefox make a correct manual backup?

The bookmarks are in the same places/folders on both the Windows and Mac, but the auto-backup problem persists. If I look at the backup files under "restore" on the Windows laptop it only shows 5 bookmarks. On the Mac, there are about 150. I did do a manual backup on the Windows laptop and that works as it should, so the problem seems to be isolated to the auto-backup.

Are you closing Firefox on the Mac via Exit because a new backup is only created when you start Firefox and not when you keep Firefox running. On Mac you can close all open windows, but keep Firefox running with the menu bar visible or minimized in the Doc. Make sure to use "Quit Firefox" with all windows still open, especially if you want to use "Restore Previous Session". See also the services.sync.engine.bookmarks.validation prefs in about:config.

Yes, I close Firefox on the Mac and then "quit" Firefox to keep it from running in the background, but the auto-backup problem on the Windows laptop persists. If I look at the backup files under "restore" on the Windows laptop it only shows 5 bookmarks. On the Mac, there are about 150. I did do a manual backup on the Windows laptop and that works as it should, so the problem seems to be isolated to the auto-backup.

Did the manual backup have all of the items or only 5? The 5 "items" could just be the folders (root folder + menu/bookmarks/other/mobile) without the bookmarks themselves. I doubt there is a setting to accomplish that. More likely there is something unusual in the local file(s) places.sqlite and/or favicons.sqlite. I can't think of anywhere to check for a possible error message in the automatic backup process -- it would be very lucky/coincidental to have the Browser Console (Ctrl+Shift+J) open at the moment that function runs.

Could you let us know what you get when you run the "Verify Integrity" diagnosis and repair function. That's here:

Open Firefox's Troubleshooting Information page using any of these methods:

• "3-bar" menu button > Help > More Troubleshooting Information
• (menu bar) Help > More Troubleshooting Information
• type or paste about:support in the address bar and press Enter

Near the bottom, there is a section titled Places Database -- that's the file which stores history and bookmarks. Click the button labeled Verify Integrity. There may be a 10-15 second delay before results appear.

If all is well, the report that appears below the button should start with:

> Task: checkIntegrity
+ The places.sqlite database is sane
+ The favicons.sqlite database is sane
> Task: invalidateCaches
+ The caches have been invalidated
> Task: checkCoherence
+ The database is coherent

Does yours say that, or something else? You can select and copy the report, and then paste it into a reply for review and comment.

See also "View Statistics" directly under "Verify Integrity" in about:support.