When you are on a site that wants to use Flash and Shockwave Flash is set to "Ask to Activate", you should see a notification icon toward the left end of the address bar and usually (but not always) one of the following: a link in a black rectangle in the page or an infobar sliding down between the toolbar area and the page. The plugin notification icon in the address bar typically looks like a small, dark gray Lego block. When the page wants to use a blocked plugin, the icon turns red to alert you to the concern. If you see a good reason to use Flash, and the site looks trustworthy, you can go ahead and click the notification icon in the address bar to allow Flash. You can trust the site for the time being or permanently. But some pages use Flash only for tracking or playing ads, so if you don't see an immediate need for Flash, feel free to ignore the notification! It will just sit there in case you want to use it later. ---- Also, it might not be a Flash problem. Many sites have switched from using plugins to using the built-in HTML5 video player. However, on sites that use proprietary encoding such as H.264 (MP4), Firefox needs external software to decode the video. Fortunately, Apple supplies this software with current OS releases, so normally it just works. Could you tell me what boxes are blue and red on this test page: https://www.youtube.com/html5