More widely supported Reading Mode. People piss on Safari and love to hate it on HN and Reddit but the Reading Mode has saved my eyes many, many times.
Additionally, something that will make me immediately switch to any browser: ability to style websites even if they don't have switches for light/dark mode. Put that in a browser and I'll switch right away. (Although this can be easily tackled if the Reading Mode works on the site, so it's related.)
Finally: built-in, harsh, and highly configurable ad-blocking.
If you have examples of sites where Firefox's reading mode doesn't work, you can file bugs here: https://github.com/mozilla/readability