Hi there,
The method linked by Zilupe described in the note is for Windows IE only, probably not good enough.
I haven't tried this myself and for the comments on the page it seems it's not reliable, but check this out: Firefox Focus and Actual Links.
This is a very annoying problem that effectively kills any accessibility efforts you might have put in your Flash application. Blind users using non-IE browsers will NOT be able to use your app because they simply cannot click on it to give it the focus.
Last time I investigated the problem I found a comment by some FF developer stating that it was actually Mozilla's fault (sorry, lost the link) but I honestly don't care. Adobe should chase up Mozilla and put some effort to fix this, it's on Adobe's interest.