Unless I’ve missed something, FF5 is as standards-compliant as the earlier versions. It probably isn’t the browser, at least not the proper version. Without knowing more, I’d guess you have a corrupt installation. You might try deleting it and reinstalling it (back up bookmarks and such). Or for some reason I wouldn’t attempt to explain, the user settings got reconfigured to turn off CSS, Flash, and JS.
Try disabling any plugins and addons that you have installed one at a time, it may be a plugin that’s not fully compatible with FF5. Are you able to view any flv (flash video) files? Try downloading the latest version of flash for ff