A friend of mine https://twitter.com/duckpoultry suggested I might like this feature. and pointed out that opera supports it: https://blogs.opera.com/mobile/2015/12/force-enable-zoom-opera-34-for-android/ He's right, I do like it and I'd love it if firefox supported it.
I tried it just now in Opera and reflowing text with pinch-to-zoom is quite nice. I tried my own blog and news.com.au. If I added a plugin for inverse-high-contrast like I have in FIrefox it'd be amazing, and more convenient/quicker than Firefox's reader mode.
I've re-opened this bug because the firefox behaviour and Opera behavour are different, and the opera behaviour is better (in my oppinion as a user). I'm expecting people to disagree with me from the perspective as web developers or something about standards or what-not, and that's possibly legitamate, but please do try Opera and see if you like that behaviour better as a user. Ask someone with poor eyesight like a senior citizen what they like.
See Bug 710298 for the original feature that is now broken. The comments on that bug since it was resolved provide other reports of the regression.