firefox for os x opacity bug!
I know this has been documented before, but I just ran into it.
Basically, when you set the opacity on some object on your page, firefox for OS X switches rendering modes - which switches anti-aliasing modes - which causes text that gets redrawn to appear slightly different.
I ran into this when working on my cool "scroll/fade" effect for showing comments. When the comments would fade in - just in firefox on OS X - the whole page would get redrawn in a different rendering mode and look "different." (screenshot grabbed from mezzoblue.com)
Naturally this sort of sucks. What good is a fancy effect if it causes the whole page to look like crap for 1 second and then look good again?
The generally accepted solution (which I am currently using) is to set all text that could be affected to have 99.9% opacity. This causes it to always use the different rendering mode. Unfortunately, it also causes the web browser to do alpha transparency calculations for almost every item drawn on the page, causing all rendering to take considerably more time. This isn't a problem in most cases, but the "scroll/fade" animation suffers quite a bit.
What a pain in the neck.
UPDATE: I'm just ignoring this problem for now. Firefox 3 is about to come out and that should erase the problem. Mac+Firefox users will upgrade fairly quickly. Ugh.