Firefox does not have an inbuilt method to tell you statistics about your bookmarks so we have to use a workaround to get the bookmarks total, but tankfully it is very easy to do.
- Open the Bookmarks Manager (Library) using Ctrl+Shift+B or go to the menu item (Bookmarks / Show All Bookmarks)
- In the search box at the upper right of the Library dialog, type http - This will match pretty much all your bookmarks in a search result which now lists all of your bookmarks
- Click on one search result so it is highlighted
- Press Ctrl+A which will select all of the results and the number in the bottom bar will tell you how many results there are. The result is the number of bookmarks you have in firefox.