When did you discover this? Were there any problems at the time? Updates? Anything?

1) Firefox automatically makes backups of the bookmarks. These are stored in the bookmarkbackups folder.

Open the Bookmarks Manager; Press the Alt or F10 key to bring up the toolbar, and select Bookmarks. Hot key is <Control> (Mac=<Command>) <Shift> B.

Once the window is open, at the top of the page, press the button labeled Import and Backup. Select Restore. From the list, pick the file you want and click it.

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.

https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks

2) Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

This started last Tuesday. I use Mozilla Thunderbird and out of nowhere the direction in which the mail is displayed had been reversed. Instead of the most recent incoming mail at the bottom it was now at the top; I couldn't find any reason why that had happened. Looked in help and found that I needed a certain add-on. Got that but it needed an app (free) from the Microsoft store. Downloaded that, then used them and successfully corrected email problem. Next went to open Firefox, but an info box informed me that Firefox needed to be "refreshed". Couldn't stop it. Suddenly MSN was my home page; had replaced what used to be there. Got "old" home page back then noticed bookmarks were gone. This HAS to have something to do with the download of the app needed to activate the add-on. I found the 15 backups on the "Old Firefox" on the desktop. The containing file (or folder?) is shown as 6vusnjq7.default. Once you click on that, the first entry is "bookmarkbackups. Each is preceded by the lower case "e" with the yellow halo through it, i.e., the Internet Explorer logo. I am NOT tech savvy. How do I get these back to my Firefox bookmarks? Thank you!

This started last Tuesday. I use Mozilla Thunderbird and out of nowhere the direction in which the mail is displayed had been reversed. Instead of the most recent incoming mail at the bottom it was now at the top; I couldn't find any reason why that had happened. Looked in help and found that I needed a certain add-on. Got that but it needed an app (free) from the Microsoft store. Downloaded that, then used them and successfully corrected email problem. Next went to open Firefox, but an info box informed me that Firefox needed to be "refreshed". Couldn't stop it. Suddenly MSN was my home page; had replaced what used to be there. Got "old" home page back then noticed bookmarks were gone. This HAS to have something to do with the download of the app needed to activate the add-on. I found the 15 backups on the "Old Firefox" on the desktop. The containing file (or folder?) is shown as 6vusnjq7.default. Once you click on that, the first entry is "bookmarkbackups. Each is preceded by the lower case "e" with the yellow halo through it, i.e., the Internet Explorer logo. I am NOT tech savvy. How do I get these back to my Firefox bookmarks? Thank you!

FredMcD said

Old Firefox? Look inside. Look for the folder with the latest creation date.

Finally solved! Thank you.

Glad to help. Safe Surfing.