FireFox browser renders main page with some weighted fonts too light to read

[sln-rxf]

Bug Description

Reproduction steps

As seen in the screen capture. One overlay depicts the FF font configuration check box.:
General Settings - > Fonts -> Advanced -> "Allow pages to choose their own fonts, instead of your selections above"
is checked.
The part of the screenshot under this overlay is the regex101 main page where the fonts have a very thin/light weight
that is unreadable.
When the "Allow pages to choose..." is unchecked, the page renders perfect.
This is claimed to be because of the built in font rendering engine in FF that is different than for instance Chrome.

This became noticeable about 2 months ago, presumable when the the new UI was initiated.
Previously the page rendered fine with the same FF settings that allowed the page to specify fonts.

Expected Outcome

Browser

FF version 115.34.1esr (64-bit)

OS

Windows 7 ( sp1 ) - v 6.1.7601

[working-name]

Can you throw in a sentence about the bug?

[sln-rxf]

Can you throw in a sentence about the bug?

Done.

[firasdib]

I am unable to reproduce this on FF 149.0.2 on OSX 26.3.1...

Have you tried a newer version of FF?

[sln-rxf]

I am unable to reproduce this on FF 149.0.2 on OSX 26.3.1...

Have you tried a newer version of FF?

My OS is on the 115 branch and is the latest version on that branch.
I won't be changing my OS to get an updated FF.

In the wild this bug now exists, detected when the UI just changed as stated.
Maybe you'd like to reinstall to branch 115 and duplicate the problem ?

[firasdib]

I mean, you're on very old software all around, but I am surprised by this issue.

I don't have any means to debug this as of right now, and I can't really think of what would cause it. Perhaps some font antialiasing issue?

[working-name]

I've set up a VM with windows 7 64bit sp1, and FF 115.34.1esr and while I can reproduce the issue, nothing I've tried so far fixed it. I asked chatGPT for suggestions as well. So it's not font aliasing from the OS, not font-related css rules (tried -moz specific too), not hardware acceleration (as much as one can do that with a vmware emulated gpu). So I'm out of ideas on this one unfortunately.

Chrome based browsers seem to not have trouble rendering the font on the same OS, so maybe the problem is actually the way firefox did things?

[firasdib]

The issue seems to be related to the fact that I use a variable font, which your version of Windows (and Firefox in particular) is unable to render. The result is that you get font-weight 100 for everything, which is clearly not great. The easiest workaround would be for you to override this CSS variable: --font-family-default

If I recall correctly, this can be done with user profiles in Firefox, so it should be a persistent fix for you, as soon as you have that.