I'm currently using a MacBook with Firefox 4. And I didn't install Flash. If I really need it for Piwik, I go Chrome.
Anyway... On every page that has a editor, I get this warning that states I have to install a plugin in order to make the page look correctly. Firefox is speaking of the Flash plugin because of the flash uploader. Fine.
But I today saw a WordPress which has by default a Flash-uploader as well and it was disabled (because I don't have Flash installed). So no message.
I would love to see this in XF as well.
PS: I was unsure if it's supposed to be a suggestion or a bug report... Feel free to move if you don't agree it to be a bug report