Flash at snail’s pace on Firefox
Yes, I am also one of those troubled souls when it comes to getting your project work cross-browser. It drives me crazy getting things to work in the exact same way on IE and Firefox.
Problem 1:
One of the flash intro’s I made ran very slowly on Firefox. It worked just perfect on IE. The intro was jerky and gave a feeling that something heavy was getting downloaded in the background.
Fix:
Remove “Transparent Windowless” option (if you have selected it) when exporting your flash movie and it will work like a charm on Firefox. Worked for me, should work for you too 
Problem 2:
I exported a flash movie as 100% to fit perfectly to the browser and resize as and when the browser was resized. It worked just perfect on IE. In firefox the movie got loaded as a small box. It was not even the actual movie size.
Fix:
Remove the < !DOCTYPE> tag from the page where you are showing the flash movie. The movie will now fill the browser and resize as you resize your browser.