wmode="transparent" in firefox
Well this is going to be a "don't do it!" post.
To make your flash movies transparent (so you can see the html content through your flash movie). You can specify a special param.
<param name="wmode" value="transparent" />
If you specify 'opaque' instead of 'transparent' it apparently has the same effect.
However, firefox (and other mozilla-based browsers) have serious issues with this. The most important, in some cases the flash movie doesn't respond anymore to any input coming from your mouse. So that means users cant click your buttons and what else.. This guy seems to have even weirder problems.
So if you are targeting mozilla, avoid this setting.. there are no good alternatives at this point.