If for some reasons, you want to delete all your Firefox bookmarks at once, to start afresh, follow me through the steps below:
==> Launch Mozilla Firefox on your computer
==> Click the Bookmarks icon
==> Scroll down and click "Show All Bookmarks" to open the Library window
==> Click "All Bookmarks" in the left navigation menu pane
==> Click "Bookmarks Menu" to view your bookmarks in the right pane
==> Hold the "Ctrl" key and click each bookmark you want to remove to select multiple bookmarks.
==> Press "Delete" to remove the bookmarks and then close the Library window.
Alternatively, you can start Firefox in safe mode ( aka "Restart with Add-Ons Disabled") and reset your bookmarks to default. You can try the trick below if your version of FireFox will support it.
==> Start > All Programs > Mozilla Firefox : Mozilla Firefox (Safe Mode)
==> "Reset Bookmarks to firefox defaults"
I hope this helps.
Recommended: How I BackUp FireFox Bookmarks and Transfer To Another Computer