http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox

jscher2000 has a tool on his website to extract out your bookmarks and save them to an HTML-format bookmark export file. If you want to try it, here is the page:

https://www.jeffersonscher.com/res/bookbackreader.html

To open the bookmarkbackups folder in Finder, use the "Open in Finder" button in the first table on the Troubleshooting Information page, and double-click into bookmarkbackups where you should see several dated files. From there you can drag and drop onto the conversion tool.

To import the HTML file, see the steps in this article: Import Bookmarks from an HTML file.

I exported the html under step 2, but I guess the file I was trying to open only had the five default bookmarks that come with Firefox. Is there another place I should look?

Look at another file.

The backups are stored in a jsonlz4 file named something like this; bookmarks-2016-06-29_1424_enD1GTH ya0JmicnYWg4V4g==.jsonlz4

In this example; 2016-06-29 = date created, 1424 = number of entries.

My laptop crashed on 8/23, and the only recent files like this read 2021-08-25, which is when the guy finished reinstalling Windows 10. The largest is the one I tried that just had Firefox bookmarks. It's only 1.94 KB. I thought it would be a clone of what was on my laptop before it crashed, but now that I see how small it is, it's clearly not. Is there someplace else they could be?

. . . which is when the guy finished reinstalling Windows 10

During the re-install, did he select to save your old files? Look for a folder called Windows.old in your user folder.

I don't have a file by that name. The way he described it was that he was making a clone of all of my files. All of my documents, pictures, music, all of it has come back as it was before the crash.

I called for more help.

I've seen other posts where someone says that bookmarks are also stored in places.sqlite, but they don't say if there's a way to extract them in situations like mine so that they can be used. I look forward to hearing if that file can be used to get them back.

You would still need an older backup copy of places.sqlite that would have been in the same location as the bookmarkbackups folder mentioned above, so chances are not very good.

Note that the Firefox profile folder is in a hidden location, so make sure you can see hidden files and folders.

So in the Profiles folder I double click on the default-release folder, and there's the bookmarksbackup folder in there that had the jsonlz4 file that I already tried that was very small, as I mentioned above.

The default release folder also has a places.sqlite file that's 5120KB and has a date that keeps updating to today. I have not started saving bookmarks again since I knew they would be written over if I tried to address this. The places.sqlite file has been 5120KB since I first found it several days ago, and I've done a lot of internet research in the past few days, meaning I don't think it's growing because of my history of visited websites. Can this be used to retrieve the missing bookmarks?

places.sqlite is by default 5 MB and its size is only increased by another 5 MB when there is not enough free space in the file. Like I wrote, you need to search for an older places.sqlite copy in the backup made by the guy who reinstalled Windows.

While using an old file to restore your bookmarks may not have all of your lost data, it is still better than trying to recover from scratch.

My bookmarks are back. I called the guy at the computer place I go to, and he had me come in so that he could copy my pre-crash places.sqlite file off of his computer and paste it over the one on my laptop. Thanks for all of the replies!

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems. Go to that post and click the 'Solved' button to its right.