There are two ways to remove a bookmark in Firefox or Android:

• Go to a bookmarked page.
• Scroll to the right of the page to see the navigation toolbar.
• Tap the "star" button on the right-hand toolbar to display the bookmark options.
• Press the "Remove" button.

Or

• Tap on the title-bar or press your phone's Search button to open the address bar.
• Tap on the "Bookmarks" tab under the address bar.
• Press and hold for 1 second on the bookmark you want to remove, to bring up a menu.
• Press the "Remove" button.

Click the Bookmarks button Bookmarks button ★on the right side of the navigation toolbar and select Show All Bookmarks to open the Library window. in the below figure click on delete from the list of options