I installed Firefox 3 in my Mac yesterday and I'm liking it better than version 2. It seems to be faster and it solved some problems I was having.
I discovered later that one extension I have installed, Adblock, was blocking *ALL* Flash content. I was using SWFObject's dynamic embed mechanism, so I am not even sure how it was blocking that, but it was.
The solution: completely disable Adblock. By the way, you need to disable the extension, not Adblock's disable feature, because even when disabled, it wouldn't show Flash content. There are tons of comments describing this problem in the extension's page at Mozilla, so I hope they fix it soon.
So that's regarding Adblock. But I should also add to the title Firefox + Flex = disaster, because I am running into some very strange problems with the Flex app I was trying to test. I saw the problem on other apps as well. If I have some elements in the screen that are clickable, and I click on them, I would see the html background as a square in what it looks like the Flash player redraw regions, which are approximately the size of the element I was clicking on. That would make the square flicker for a second, then I would see the normal content. It's as if the redraw region would become transparent for a second, then back to normal. Very annoying. Normal buttons work fine. Has anyone seen this?