I love Firefox, but there are a few things about it that annoy me. For example, you can’t paste the primary selection from the keyboard. I tried hard to fix this in mozilla itself, but in the end, I failed. :/
However, I found a great program called autocutsel, written by Michael Witrant, which synchronizes the cutbuffer and an X selection. With a little help, it now synchronizes both the clipboard and the primary selection. Problem solved!