Sizing question. Firefox and IE

I’m mainly a video maker but have used Flash many time for illustration and animation, I’ve started using these skills for making websites, I understand basic actionscript and have got loading layers, controling other clips and php mailer sorted, but I’m definatly a complete novice knowing what flash its capable of. The main problem with my sites are accessibility, I know there is a guide on the macromedia site but its way too long and I only have one major concern/question, Sizing. (And a couple of related questions)

Using the flash export settings under ‘html’ to ‘scale’ the movie or using ‘dimensions’ set to 100% using any settings the outcome is distorted when opened in firefox. Is this a firefox bug? It seem unlikley as it is now so widely used. If not what is the correct way to export or package my movies so they open at the optimized size in IE or firefox?

Also, some people tell me they need to ‘activate’ the flash in IE with a frame around ther movie. I know this is something to do with accessability ‘control’ but some browsers don’t show it, looking at the advanced tab under accessibility I couldn’t see anything to change. Using IE on my mums computer showed frames to ‘activate control’ but with the same settings on my brothers computer using IE there was no problems.

Thanks in advance for any help with these issues