As of Safari 5, Safari "unofficially" supports extensions now. It is just, Apple didn't setup an "app store" for them. So, no need for "input managers" or "saft" like applications, Safari will be able to run Firefox style extensions soon.
As it runs under Windows, it will be easy to test/develop too. Just reminding if you don't follow OS X scene or the iPad etc. noise is too high to follow
