For the impatient:
Patch for autocutsel 0.7.0
Autocutsel page

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!

See also:

• synchronizing GNU screen's paste buffer and the X selection
• why I don't run shells inside Emacs