Does it not work for you on Linux? When I use Firefox on Linux, the reader view icon shows up for me at the right end of the address bar on supported sites.
It even works on HN discussion pages (though HN is such a simple site that there's no benefit). You can test it on this discussion (https://news.ycombinator.com/item?id=30061672) or any Wikipedia page.
Mozilla's reader view is actually one of the browser's strengths compared to their Chromium competition. Firefox offers a reader view far more often than Chromium-based browsers do, and probably more often than even Brave offers their new Speedreader. It's one of the things I miss from Firefox for sure.
If it was working correctly though. On the desktop version it seems to be working almost flawlessly, but on mobile it requires an internet connection when you're reopening a tab in reader-view mode which is a bit odd. I'm starting to thing this is probably due to the existence of pocket integration (there's an old bug report).
And the fact that we are connected 99.9% of the time doesn't make it less annoying on the next flight (whenever that will be in this pandemic :)).
And those Pocket ads.
And why isn’t there an easy reader view at one click?
And why do they make it so hard to disable JavaScript? In Safari it’s just one easy key combo.