You'll have to do it one by one at least the practice I did.

Tap and hold the bookmark you want to delete. Then tap Remove. Hope this can help you

How were you trying to remove the bookmarks? The Bookmarks Manager creates backups automatically.

These instructions will remove all bookmarks and history from your profile. If there is something you want to save, create a backup first.

Creating a backup file. 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 Export Bookmarks To HTML, and follow the prompts and save it to a HTML file. Copy the file to another computer/profile. Repeat the instructions above, BUT select Import Bookmarks From HTML.

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

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

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.

Delete or rename the places.sqlite file: History and Bookmarks. Also delete the bookmarkbackups folder.

When you restart the browser, new, clean files will be created.

You can delete (rename/remove) places.sqlite to remove all bookmarks. The places.sqlite database stores bookmarks and history, so you will lose the history this way.

If you do not want to lose the history then try to select all the bookmarks in the Library. Open the "All Bookmarks" folder in the right panel and select all the bookmarks and folders via the right-click context menu and press the Delete key or use the right-click context menu to remove the selected items.