Sorry, I forgot to mention this:

Firefox won’t start with my designated homepage. Everytime I open Firefox, I get three tabs - Welcome to Firefox tab, Firefox start page and a tab concerning a Firefox extension being updated. I did get a fourth tab, about another Firefox extension being updated, but removed it. It no longer appears now.

The preferences are set to default and they won’t save the changes made to them (well, they initially seem to), thus having the same problem every time I open Firefox.