Have noticed that when viewed in Firefox 3.6 some SWF files seem to mysteriously resize themselves or show a focus border when clicked on. At first I could not figure out what was happening when my fullscreen 100%x100% SWF suddenly forced scrollbars to appear in the browser once it received focus. It turned out that this is due to the fact that Firefox 3.6 now draws a 1 pixel 'focus' border around the Flash content which is very annoying. The same happens on Java Applets too.
Luckily there is a fairly easy solution to this, and itinvoves a bit of CSS. Simply add this to your page's stylesheet:

view plain
1a:focus, object:focus { outline: none; -moz-outline-style: none; }

Problem solved.