I’m guessing you have already figured out how to delete one or two bookmarks using the Bookmarks > Organise Bookmarks option from the FF toolbar.
If you want to delete several at once you can use the Organise Bookmarks option. Click on the first bookmark you want to delete then move the mouse to the last one you want to delete. Press the Shift key and click on the last one. They should all now be highlighted. Hit Delete.
If you want to delete all bookmarks from Firefox all you need to do is start Firefox in Safe mode.
Start > All Programs > Mozilla > Firefox (Safe Mode)
This launches the following box:
Choose the option to Delete All Bookmarks and then Continue in Safe Mode. The next time you start Firefox all the bookmarks will be gone.