Just enter about:profiles, the rest will be pretty straightforward.

Thi word "straightforward must have a different meaning where you live. I found it, but what do I delete ?

You can delete places.sqlite (bookmarks and history) and the automatically created backups in the bookmarkbackups folder (compressed .jsonlz4 JSON backups) with Firefox closed to start with fresh bookmarks. Note that you lose the history when you remove places.sqlite. See also:

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

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

Maybe you already tried this method, but just in case: you should be able to delete your Firefox bookmarks from the Library window. Here's how:

(A) Launch the Library window using either:

• Ctrl+Shift+b
• "Show All Bookmarks"

Firefox should open a new window listing the contents of the "Other Bookmarks" category.

(B) Select everything listed on the right side using either

• Ctrl+a
• single-left-click the first item then press Shift+End to select to the bottom of the list

and then delete using either

• the Delete key on the keyboard
• right-click > Delete

The bookmarks should all vanish.

(C) In the left column, click Bookmarks Menu, then repeat the steps under B

(D) In the left column, click Bookmarks Toolbar, then repeat the steps under B

That should get rid of them.

Note: That doesn't remove backups.

If you have (ten) thousands of bookmarks then you should be cautious with removing them in one go in the Library.

Note that there is a third bookmarks root folder (Other Bookmarks) that you would have to clear as well.