Update Firefox logos and screenshot quality

(Bug 881207)

Updated the Firefox logos, and in the process improved the scaled screenshot quality a bit (and a lot for OS X). I didn't have a PSD original for the awesomebar*.jpg images, so they are JPG images re-saved as JPG again, which would eat some quality, but they do look ok.

The features*.jpg images came from an updated PSD, so they are good.