Sadly, development of reader mode seems to be stagnant for both Firefox and Chrome. While it works for a substantial number of pages, I was hoping that more pages would work as years go by. Too bad that doesn't appear to have happened.
One interesting thing I noticed after updating to iOS 18 specifically, is the removal of reader mode as a "mode". It's now OFFERED to you in the new view menu based on some heuristic about the page that Apple has determined, but it's not a mode you can turn on regardless of if Apple thinks you're on a "readable" document or not.
Frustrating since their "remove distracting elements" feature was a great value-add, done in the same update.