If you're using a laptop, you probably don't have a discrete GPU—not if you care about battery life. But you probably still want to play videos without dropping frames. All laptops ship with some form of integrated graphics on the CPU, and the vast majority of modern (and even not-so-modern) CPUs support some level of hardware decoding. Here's a good place to start to get hardware decoding working.
Firefox allows the user to create a userContent.css file that it loads for every page. The trick, however, is that you write CSS queries that only act on certain domains/websites/webpages to redesign it at will. Think of it like going into the Developer Options and changing some CSS rules, but this way, they stick every time you browse to that webpage again.