Also when I want to add a bookmark, I can only put it into one of the first five folders and not the rest (about 20 folders in all). . . . . . . . I have tried re-installing Firefox but this did not help.

I guess either no-one reads these messages or Firefox support doesn't give a darn about their customers.

I had the same problem and just figured it out. When you hit "bookmark this page", a box opens up. Where it says FOLDER, there's a box that says "bookmark menu" with a little graphic to it's immediate left. at the far right of the box, there is a down arrow that opens a menu box that shows the first 5 folders you had mentioned. but there s no need to open that menu. Just to the right of that down arrow, there's another down arrow, and if you mouse over it, it says “show all the bookmarks folders”. If you hit that, an expanded bookmarks menu box opens and you can see 6 folders immediately, and can see the rest by using the slider at the far right. Hope this helps and hope I've explained it well enough, I'm not very articulate and have trouble explaining things at times.