User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:100.0) Gecko/20100101 Firefox/100.0

I enable full hinting and subpixel AA in the system settings to improve the readability of my fonts. Despite that, Firefox 100 on Kubuntu GNU/Linux 20.04 doesn't display web pages' content using subpixel anti-aliasing on certain sites.
I met with this issue for the first time when I switched from ESR 78 to ESR 91. There exists an option gfx.webrender.force-disabled in ESR 91 which helps to cope with grayscale AA on most pages except the most stubborn like reddit.com.
But in the newer Firefox versions this option is absent.

Actual results:

I'll provide several examples. One of them is https://askubuntu.com. As you can see on the screenshot I attached ESR 91 displays it with subpixel AA, but version 100 uses grayscale AA.

This is mostly inappropriate on the pages which use small font sizes. For example, I attached another screenshot of Roundcube web-mail, displayed differently by 91 and 100 versions.
Moreover, on the same page Firefox 100 displays the "Folders" list using subpixel AA, but the letters list is displayed using grayscale AA so this distorted text looks more evident in comparison.

Expected results:

Maybe this case is not that obvious on HiDPI screens, but for the majority of users with the regular screens or laptop screens, for visually impaired persons, eye fatigue is a significant issue.
Is it possible to have subpixel AA enabled everywhere, despite possible performance hit? Healthy eyes are more important.

Whtyger, please attach your about:support output to this report when you get a chance.

I also observe the same issues on certain sites, especially on web apps like Notion (https://www.notion.so) or Capacities (https://app.capacities.io).
Note their homepages usually render with subpixel AA, but after you log into the actual app (which is probably a more complicate page), it turns into the greyscale AA, which really hurts my eyes.

The only workaround for me was to disable the Webrender completely, but since that option is gone, I had to install older version 91 ESR where I'm able to disable the Webrender again by gfx.webrender.force-disabled = true, which only is able to render text with nice subpixel AA.

PS: I don't understand why the option to disable Webrender was removed when there are still such issues with text AA. (I even tried the latest Firefox Nighty 113 just to find out it still produces grayscale AA on those sites).