calc: Bump to 2.12.4.8
firefox-extension-itsalltext: Add PKGBUILD

I have only tested this on Firefox 22. It may work with older versions,
I don't know.
firefox-extension-overbite: No need for "|| return 1"

Minorminorminor change, no need to bump pkgrel.
firefox-extension-*: Remove versioned dependency

Users of the AUR are expected to have a fully upgraded system.
firefox-22.0-1 is in "extra", so everything's fine.