I suspect it is a bug.  I looked at the code for the main page and didn't find a single flash extension.  It is possible, however, that I missed something but I don't think I did.

My main systems are fairly tightly filtered so not seeing anything at the moment, but I recall having noticed something when using other systems... though I rarely visit the top level page -  just jump direct to forums.

Possibly related to an element from addthis.com, there's a javascript object that probably has it embedded: addthis_widget.js
When allowed,there's also something called from clearspring.com (button1.6.swf), tho the loaded flash object doesn't seem to actually do anything onscreen. (Firefox media info reports it as 0x0 pixel and 1425 bytes.)

The text around this in the html source implies it's related to current development with the friends list and iphone version.

When I First set up my latest Virus/Firewall programs many months ago , Invelos popped up and I asked me if I wanted to always accept this web site for IE to accept it., either always, just this one time, or Never .
, (as Invelos can tell,/report - don't know how, -- when you are logged in..,)
That's how Invelos can give you any updates when it was going thru it's beta  versions and was updating to the newest beta along the way, a few months ago..,
Any way,., I suspect Firefox is asking for the same type of acceptance of the Website as IE7 does ..