Here's a really weird bug: As of today, you can no longer type a space in YouTube on any legacy-Firefox-derived browser, in either the search box or when typing a comment -- it's reserving the space bar exclusively for pausing/unpausing the video. Typing Alt-032 to generate ASCII 32 or pasting in a space character from the clipboard works, but pressing the space bar doesn't. I've tried Serpent/Basilisk 52.9.0, New Moon 27.10.0, and Mypal 28.14.2 on XP, and Firefox ESR 52.9.1 on my Mac, and they all do the same thing. The space bar works normally in Chromium-based browsers.
Edit: I just updated Mypal to 29.1.0 from their Github page and now the space bar works in YouTube. So it appears that Pale Moon 29.x-based browsers should not have this problem.