Open more than 50 tabs in firefox. Try to bookmark open tabs by "Bookmark All Tabs". Then check the bookmarks which firefox created. Some open tabs are missing in bookmarked list. I've experienced this problem multiple times.
Re: bug in bookmarking open tabs in firefox 23.0.1
@cor-el: "Firefox will only bookmark a specific URL once"
bookmark a specific link. then open in in firefox with some other tabs. booksmark all open tabs by "Bookmark all tabs". check bookmarks. you will see that twice (and even more) bookmarking of same link is possible.