Please provide steps to replicate the issue. What steps are you taking to "preview" the pdf? Does this mean that you're simply viewing a pdf in Firefox? Where is this file, can we access the same pdf? Do you have the same issue on this page? --> https://www.irs.gov/downloads/irs-pdf Did you try Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#search104

@jonzn4SUSE

After I reset my firefox to factory default state, the link from https://www.irs.gov/pub/irs-pdf/p5651.pdf works fine. But after I drop my user.js file into my profile, the font becomes weird again.

user.js: https://github.com/AspiringZero/FireFox_with_TST_Setup/blob/main/user.js

I would start by disabling the "GFX" section of your user.js.