Revert "Firefox 20 reference in release.nix. Fixing."

This reverts commit 19dd9dc877150f74be172409132855c190b6d778.
Revert "Fixing firefox reference"

This reverts commit a17b45686b3f1ad656771848ab5bf3bc3c758e2a.
Revert "firefox update to 21.0"

This reverts commit 971337a119d708d78a152f4d918694e7b5a2b7e8.
Reverting firefox to version 20

Firefox 21 builds but does not start. I have no idea of what happens.

Merge branch 'firefox_revert'