Reader mode sometimes doesn't appear for some sites. I am guessing there is a way for sites to disable it with JS or indicate to the browser that they don't want it.

It's a feature of Reader actually. The library they use has a "isProbablyReaderable()" function to determine if a page can be simplified, and their extension uses that to decide if Reader mode should be available or not.

[0] https://github.com/mozilla/readability/#isprobablyreaderable...

And reader mode doesn't auto activate during page load...

If I have to wait for all the ads and banners and fonts to load anyway, most of the benefits of reader mode are removed.

Sometimes Reader mode gets around adblock detectors or other stoppers on normal page loads. It'd be nice to be able to try it regardless of whether Mozilla thinks it would be useful.