Well I upgraded from Firefox 3.* (last one before 3.5 afaik) to 3.5.2 today and now nothing using Flash works. Any site that uses Flash says that I either have Javascript disabled (it's enabled) or an old version of Flash and directs me to a page to download the latest version. I jump through all the hoops it wants to install it and it still won't display any Flash. Any ideas?

Obligatory question: Have you checked whether or not you're using any extension which could cause the issue?

Fixed it. All the plugins were disabled.

Was checking my extensions again and noticed the "plugins" tab. Never noticed it before (or not paid any attention to it).