YouTube is a great platform, but it certainly comes with its fair share of annoyances. One of the most sought-after YouTube hacks is a simple one: how do I play YouTube videos in HD by default?
Most of the time, YouTube automatically detects the strength and speed of your internet connection and plays videos in what it deems an appropriate definition. When it errs, however, it errs on the side of speed, meaning that YouTube would much sooner load a lower definition than your connection is capability than load it at a higher definition, lest you encounter stuttering or otherwise slow playback.
Plenty of hacks exist, but if you're running Firefox, there's no need to get your hands dirty. Just download YouTube High Definition and videos will load at the highest resolution possible thereafter.
Normally, I wouldn't advocate installing an extension when a tweak or behind-the-scenes trick would do, opting to keep your system resources at a minimum. This particular extension, though, seems to do the best job of solving the problem it attempts to, while staying quite light on resources. In fact, as I write this, the entire first page of reviews are five-star ratings... so don't just take my word for it (but I'll say it again: it's simple, fast, light, and effective).