Something happened to Firefox on my computer - all bookmarks ditched, password registry blanked, all settings restored to default, etc. Pretty much effectively a fresh download. I'd like to restore it to where it was before Mozilla trashed it, but failing that...

One of the things I'd like to do is restore my choices as to how it handles file types. The present list is four file types, and there seems to be no way to add new ones. There is a search box above the list, but it does nothing. Type a suffix in it and nothing happens. Press [enter] key and nothing happens. There is no action key or [search] button. So how does one add a new file type to the list?