[System] Font rendering issues in Firefox (and GTK apps)

[apogiee]

Firefox - and all GTK apps in general - seem to ignore the KDE sub-pixel rendering settings, and fontconfig on Wayland. GTK apps use grayscale font rendering instead, resulting in a noticeably worse experience making all fonts seems fuzzier and blurrier at the same time compared to how crisp they appear in Cinnamon (presumably Gnome too). Screenshots for comparison: https://imgur.com/a/H8h1Oeb

After hours of trying to fix this I found that gsettings is forcing grayscale by default:

gsettings list-recursively | grep alias

Changing that to rgba does help a bit:

gsettings set org.gnome.desktop.interface font-antialiasing rgba

But this only affects some GTK apps, and in case of Firefox it only changes grayscale rendering to rgb for some websites. Many others, like Reddit for example or even this forum, are still stuck at using grayscale even with this settings. Sites, that do render properly with rgb sub-pixel rendering under Cinnamon.


Is there any way to fix this properly, and to ensure proper rgb sub-pixel rendering is used under Wayland?

[Princey]

Question, are you using the default install (snaps) of Firefox?

[apogiee]

Question, are you using the default install (snaps) of Firefox?

Yes, this is with the default snap install. But I also managed to install and run a flatpak version, and it behaves exactly the same. And as a matter of fact you can easily reproduce this issue just by running the live session, loading any web page with Firefox and taking a screenshot of some text. Zooming in you should see grayscale is being used.

[Princey]

I do not use snaps at all. First thing I do on a clean install is remove snaps. I ran the native version (deb from mozilla) of Firefox so I don't have that issue.

[apogiee]

I also tried installing the native Firefox package, but the font rendering remained the same. It behaves identically to the Snap and Flatpak versions.

When I force RGBA antialiasing via gsettings, websites that switch to RGB rendering display fonts correctly, giving a similar experience to what I see in Cinnamon. However, sites that continue to use grayscale (which, based on further testing, still appear as grayscale in Cinnamon) look noticeably more polished and crisp there compared to KDE. I observe the same behavior in other GTK apps as well that keep using grayscale, that those fonts are rendered much better in Cinnamon than in KDE.

Based on this, it seems that the core issue is that GTK apps using grayscale aliasing render fonts much fuzzier and blurrier under KDE than under Cinnamon, to the point where it is unconsciously tiring to my eyes. Tried other KDE distros, and this seem to be a generic issue across. Despite spending hours of googling I have not been able to find any solution how to make these fonts stuck on grayscale look closer to their Cinnamon counterpart.

[apogiee]

Did a clean reinstall because all the changes I made probably affecting things now. Based on that my new findings are:

• Snap version of Firefox by default only uses grayscale aliasing, disregarding KDE settings. However, with the change gsettings set org.gnome.desktop.interface font-antialiasing rgba it does start to use rgba sub-pixel aliasing, but not for all webpages (in this the version on Cinnamon behaves the same). But where it does use grayscale, that looks worse on KDE than on Cinnamon.
• Flatpak version only uses grayscale and disregards even the gsettings changes. I'm not sure how I managed to make it use rgba, but it did work on my previous install somehow, but don't know what exactly fixed it.
• The native version of Firefox does use rgba, even without the gsettings change. And with a clean install now, the sites/sections where it uses grayscale, it seems to be pixel perfect with how Cinnamon is rending it.

So, the root cause, at least in Firefox, seems to be issues with both the Snap and Flatpak versions. However, I'm now left with another issue on the native Firefox, it is not able to work with my YubiKeys.