Skip processing and output when Firebug impossible for User-Agent

Drupal For Firebug runs its hooks and outputs Firebug info whether the user agent has Firebug or not. Originally I was going to propose that it check whether Firebug is enabled on the client, but I can't think of a way to do that. (I thought Firebug would include a identifier in the User-Agent header of GETs, but I don't see it.)

So instead, what if Firebug just checked the User-Agent header for Firefox. If it's not Firefox, don't bother. I like to leave DfF on for when I'm debugging in Firefox, but sometimes I switch to Safari. It would be nice to have that implicitly disable DfF so I can see normal page load times. Maybe not a huge benefit, but the fix is simple and shouldn't have any negative impact.