First there was the project to get Firefox to be a total drop-in lookalike for Internet explorer, and now there’s FirefoxOpera, a project that makes Firefox look and almost like Opera.
Also of note: Safari-Look for Firefox coupled with Fusion
(for the progress bar in the URL area), Firefoxy (for nicer form wigets), and Metallifizer for the window border elements makes Firefox resemble Safari a good bit.
Of course, there’s always Industrial for Firefox which makes Firefox look and feel like a GNOME application. (A theme by yours truly — and I even talk about it on Linuxart.com from time to time…)
All of this goes to show how flexible Mozilla Firefox can be.