How Can I Improve the Font Rendering In Firefox

This won't help many people, but ...

For me it was some ancient Adobe.pfb files in my .fonts directory. Specifically Helvetica.pfb .

If you have Adobe Postscript files from the 90's laying about, have a look with:

fc-match -s Helvetica | grep -i Helvetica

and see if anything lurks. I 100% would have expected @Perlence 's answer to fix this, but apparently .PFB is an exception. I don't see this problem in Chromium-based browsers, so maybe Firefox is handling it?

Anyway, the good news is as soon as I removed it, I was able to switch back to my test tab (archive.org) and as soon as Firefox reloaded the DOM it was perfect. In previous iterations I quit Firefox, logged out of my X11 session, rebuilt the font cache, etc. but none of that was ultimately needed.

See here for a before and after for identification/comparison:

I have tried the suggestions above with Firefox 75 and KDE 5.6 (Arch based Antergos linux) with no success or only slight improvement. Only after changing the Firefox font settings to Fira Sans and the fonts in Firefox gui (tabs, menus, etc) finally started to look sharp, almost as good as in Chromium based browsers. I could not replicate this with any other font no matter how I've set hinting and aliasing.

Menu -> Preferences -> Language and Appearance section, and there I changed teh Default font to Fira Sans. You can play a bit more with the Advance section. You can try Fira Mono as monotype font etc.

These sites use Helvetica as their font. On most Debian systems it's not an outline font, but raster one. You can disable bitmapped fonts as the last stage of font configuration dialog:

$ dpkg-reconfigure fontconfig-config

I've improved mine these days using fontconfig-infinality. Looks great!

The first topic of this forum shows how to do it. Do not use the first method of the link(compilation), use the deb files below it. Install with

dpkg -i *.deb

and then

bash /etc/fonts/infinality/infctl.sh setstyle 

choose option 2 (infinality)

DONE!

I've had this issue for ages, maybe it's time to do something about it!

It comes done to ClearType, Microsoft and patents from what I read. Most *nix distro's disable any patent protected font rendering by default.

Read about Debian and fonts here, you want Subpixel-hinting and Font-smoothing section.

There's a config file on that page but I will add here for future reference. Create a file called .fonts.conf in your home directory, and add the following:

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
 <match target="font">
  <edit mode="assign" name="rgba">
   <const>rgb</const>
  </edit>
 </match>
 <match target="font">
  <edit mode="assign" name="hinting">
   <bool>true</bool>
  </edit>
 </match>
 <match target="font">
  <edit mode="assign" name="hintstyle">
   <const>hintslight</const>
  </edit>
 </match>
 <match target="font">
  <edit mode="assign" name="antialias">
   <bool>true</bool>
  </edit>
 </match>
  <match target="font">
  <edit mode="assign" name="lcdfilter">
    <const>lcddefault</const>
  </edit>
  </match>
</fontconfig>