Hi   !
To delete a lot of bookmarks at once, use the Library dialog. Either:

Ctrl+Shift+b Bookmarks > Show All Bookmarks

In the left panel,   click   'Bookmarks Menu'.   In the right panel, click the top item you want to remove,  then press   'Shift+End'   to select all the way to the bottom,  then right-click > 'Delete'

(I haven't tried this myself,   as I don't want to delete my bookmarks, so:   fingers crossed   !)

https://support.mozilla.org/kb/how-do-i-delete-bookmark

I will give it a go. Thank you for your support much appreciated

Let us know how it worked for you,   will you   ?
Good luck   !