Bulk ?

You probably have some help in the management of your bookmarks by opening up the bookmarks library feature. You can then block groups of them and delete at will.

Menu > Bookmarks > showall

Tip: You might want to export your bookmarks to htm file and save to disk. This way you have a backup and able undue a mistake or two.

The System Details list shows you use a very old Firefox 69.0 version.

Doesn't your Ubuntu distribution support more recent Firefox versions ?

Verify that you meet the System Requirements (GTK+ and GLib) for the current Firefox release.

• https://www.mozilla.org/en-US/firefox/system-requirements/
• https://www.mozilla.org/en-US/firefox/86.0.1/system-requirements/

• https://support.mozilla.org/en-US/kb/bookmarks-firefox
• https://support.mozilla.org/en-US/kb/how-do-i-delete-bookmark