I updated Firefox. It said this update was speeding up the loading of pages. Yay. (Logging in to Firefox was indeed faster.)
I tried Speedometer 3.0 as it suggested, and my score was 6.25 out of 140. (I'm putting it here since I'm not on X.)
For reference: Everything (eg. opening What's App) on my computer is incredibly slow loading when I first come on, not just Firefox. It becomes even slower if I "like" a video which is counter-narrative I've noticed. When I was too busy for months to watch videos, it was neat how well my computer worked.
