The following issue reproduces for me on OS X in the latest Nightly. Windows seems unaffected.

STR:

1. visit for example https://fs.blog/2020/08/observer-effect/
2. open the print dialog
   2a. [optional] if you have the doorhanger, select "print using system dialog"
3. Select "PDF -> Open in Preview" or "PDF -> Save to PDF"
4. Look at the result

ER: Text should be smooth
AR: Text seems to be rasterized at low quality, so the result is very unsharp.

Regression window: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=432e42cbbc4139efd2319952ec33efe8ff9add3c&tochange=35cdd8678bbfed2d652deb2d7105494e366f1a0e

(Sorry, I can't narrow it down further because for some reason mozregression keeps dying on me)

See screenshot attached. Website on the left, PDF on the right.

This appears to be a duplicate of something I noticed from within PDF.js: bug 1660911. I'm going to consolidate the bugs over there. Any time you export a PDF or print a PDF using Black and White mode, you will notice this problem.