I don't think there is a single solution and this is a problem that lots of people complain about. It appears based on tests run by Jan Ozer that Safari is more efficient than Firefox for playing flash videos and upgrading to Flash 10.1 is better for all browsers.
There are even bigger CPU savings if you can find the videos delivered in H.264. YouTube, for example, has a beta test that lets you sign up to view videos in H.264. Of course only browsers that support HTML5 and H.264 (e.g., Safari and Chrome) can do this.
Like you I use Firefox and it is a pain to switch to another browser but if I am opening the browser just to watch a long video sometimes it is worth it.