But in real life as a developer, not all of your users are using the latest one. If you depend on a certain feature, it’s a pain after looking up the version that started supporting it, to then have to look up what time that number corresponds to. (e.g. “that’s last month’s browser” is a bit different from “that got fixed in the browser two years ago, please update”).

In real life, developers use https://caniuse.com which tells you exactly what features are supported in what version and when that was released.

yea. you support features / ages / percentages, not version numbers. whether there are 2 or 20 doesn't really matter.

A moment where version numbers are relevant is when you collect telemetry from your users and when you reccomend supported browsers.

I think it is in part a difference between websites oriented to the public and websites oriented to companies, where browser versions can be more complex than a self-updating browser.

Sorta. Though in that case, more versions is more better - it gives you more precision on the up-to-date-ness of the browsers people are using. Rather than saying "80% of people use a browser from between 2016 and 2018", you can say "75% use one from late 2018".

You are aware of the long term support firefox esr, right? Because the existence of that invalidates the "only the latest gets security fixes!!!" argument.

looks like v68.8 is ESR... not far behind, i was hoping for v10

Not if an update renders half of your add-ons unusable. Which happened several times in the history of Firefox (I'm using it since Firefox 2).

You don't have too. There is Firefox ESR.