I cannot use this because it always resets the custom zoom level for web site. For example I cannot browse HN with the default font size, so I set Firefox to zoom 120% for news.ycombinator.com, and I don't want to have to do that each time I come back to the site.

Yes, this is by design: https://bugzilla.mozilla.org/show_bug.cgi?id=1369357

Privacy is at odds with usability, sometimes.

Yes yes I understand that :). I'm just warning that enabling it can lead to important usability issues.

RFP is great but it also heavily interferes with addons. It disables Ctrl and Alt key combinations for them, breaks scrolling and timer-based behavior, and generally renders many of the addons unusable in various ways.

That setting can cause weird rendering issues in PDFs.

That's probably due to canvas fingerprinting protections, which can be turned off by clicking on the canvas icon in the address bar.

You can create a separate Firefox profile to effectively use as your PDF reader application.