[pdw]

As a user you can only use the polyfill to replace the XSLTProcessor() JavaScript API. You can't use the polyfill if you're using XSLT for XML Stylesheets (<?xml-stylesheet … ?> tags).

(But of course, XML Stylesheets are most widely used with RSS feeds, and Google probably considers further harm to the RSS ecosystem as a bonus. sigh)

[snackbroken]

Moz also has no love for RSS, having removed support for live bookmarks in Firefox 64 (2018) and no longer displaying the RSS icon anywhere in the UI when a website has any <link rel="alternate" type="application/rss+xml"> tags. If you want to subscribe to feeds you have to jump through a bunch of hoops instead of it being a single click.

Fortunately, Thunderbird still has support for feeds and doesn't seem to have been afflicted by the same malaise as the rest of the org chart. Who knows how long that will last.