Hi royviskupic, does it work any better if you call up the Library window and try to delete it there? Either:

• Command+Shift+b
• "Show All Bookmarks" command

In the left column, click Bookmarks Toolbar, and the list then should appear on the right. Any luck?

Possibly Related Issue

There was a recent change which prevents creating bookmarks with "invalid" addresses. Any older bookmarks that couldn't be created now, such as those having a "placeholder" in the domain name (https://%s.example.com/), might not be editable or deletable. I think I read in another thread that this has been logged as a bug and hopefully will be fixed in an upcoming release.

If "Verify Integrity" cannot repair places.sqlite, rename/remove all places.sqlite and favicons.sqlite files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created

See also:

• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file
• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox