Firefox's button is flaky - it seems to disappear depending on the page. For example it appears on HN comment page, but disappears on HN's top-level page. There is clearly some sort of heuristic which maybe tries to determine if there is "main content", but it fails on many blog-like pages, like current #5 [0].

It seems that any time I find a website with a horrible design and start looking for that "reader" button, that button is disabled in Firefox. Sometimes I tolerate the design, sometimes I switch to Chrome - because reader mode works on all sites there.

[0] https://masteranza.github.io/weedout/