I think it depends on how you get the bookmark. As far as I can tell, on mobile Firefox (the mobile browser I have easily to hand), you can only bookmark the page you are currently visiting, where the default bookmark title is the title of the page. But, on desktop Firefox, you can also create a bookmark directly from a link, in which case the default bookmark title is the text of the link. This makes some sense to me, since you probably wouldn't want the act of bookmarking a link to fetch the linked page just to find out its title.