Responsive Images Not Scaled-Down Properly in Omega Theme just on Firefox
What kind of Mac are you using? For example, I have a Retina Mac. Firefox doesn't support Retina displays, so things generally look fuzzier in Firefox than say Chrome.

Also, if the responsiveness is aimed towards mobile devices, wouldn't it be better to test on those? You probably won't have a user with a desktop window of 600px wide. ;)