Let's start with; If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Type https://accounts.firefox.com/settings<enter> in the address box. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again.

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address box.

You will find Places Database near the bottom. Press the Verify Integrity button.

If any errors are reported: https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

The places.sqlite and favicons.sqlite files contains your History and Bookmarks. And it looks like they are corrupted.

Type about:support<enter> in the address box.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above files. Then rename or delete them. Restart Firefox. The browser should then create a new place.sqlite and recover your bookmarks from the backup files.

Note: Your history is not backed up and will be lost.

Are you currently using a version from the repositories of your Linux distribution? Try the Firefox version from the official Mozilla server: https://www.mozilla.org/en-US/firefox/all/

After the problem has been solved;

Restore Bookmark Backups

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. Firefox 84+ <Control> (Mac=<Command>) <Shift> O.

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

https://support.mozilla.org/kb/forum-response-recover-bookmarks-after-update

You can try Firefox from the official Mozilla server if you currently use a version from the repositories of your Linux distribution to see if it behaves differently.

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Let's start with; If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Type https://accounts.firefox.com/settings<enter> in the address box. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again. [v57+] Places Maintenance is built into Firefox. Type about:support<enter> in the address box. You will find Places Database near the bottom. Press the Verify Integrity button. If any errors are reported: https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional The places.sqlite and favicons.sqlite files contains your History and Bookmarks. And it looks like they are corrupted. Type about:support<enter> in the address box. Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox. Locate the above files. Then rename or delete them. Restart Firefox. The browser should then create a new place.sqlite and recover your bookmarks from the backup files. Note: Your history is not backed up and will be lost.

Are you currently using a version from the repositories of your Linux distribution? Try the Firefox version from the official Mozilla server: https://www.mozilla.org/en-US/firefox/all/

After the problem has been solved; Restore Bookmark Backups 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. Firefox 84+ <Control> (Mac=<Command>) <Shift> O. 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 https://support.mozilla.org/kb/forum-response-recover-bookmarks-after-update

You can try Firefox from the official Mozilla server if you currently use a version from the repositories of your Linux distribution to see if it behaves differently.

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

I was using the app's Linux Mint repo. If I use the "tar.bz2" archive in the link you wrote, I guess the updates will not be automatic. Therefore, I should try the version I have installed from the Flatpak for a while. We can assume that my problem is solved.

I am thankful to you for your suggestions and to take me time.