Fix stylesheet infestation

Okay, I'm going to start actually adhering to Feature Branch. A lot of unnecessary effort went into fixing bugs that a second pair of eyes would likely have caught.

This is the fix for the extension stylesheets leaking into other pages in Firefox. It happened because the standard method for injecting CSS applies it globally, not just to the current page. There's a new (but undocumented) method that does it just on the current page. I'm using that now.

Since the previous pull never got into master, I've left this version as 2.2.3