Wront font used for code display in Firefox

Since yesterday my Firefox uses the wrong font (SF Mono Heavy) to render code on GitHub for example in the PR review.
The text is rendered way too bold. (See screenshot below)

I have no idea why this is happening. I did not install a System or Firefox update nor can I remember making any other changes to the configuration. Also it works fine in Safari and Firefox displays all other websites correctly.

I am on macOS 11.4 with Firefox 89.
Does anybody have an idea what might be causing this issue?

I could finally resolve this by removing (disabling) the font “SF Mono” in macOS’ FontBook app.
I would think this is a bug in Firefox to select the wrong font variation…

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Appreciate you tracking this down. I took the liberty of opening a Firefox bug report for this.

And from that bug report: I had an old version of SF Mono (pre-Catalina, I think) on my computer. Downloading the latest version from Apple here and the problem has gone away.

@greimers, can you check your font version/date in the inspector panel in Font Book?

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Thanks, @wlonkly, for opening the Firefox bug!
For me it was the same as for you. I think I had installed the SF font face shortly after it was released. So I had it as a user-installed font.
Re-Installing the latest version and removing the old user font solves the issue. (It was version 12.0d2e3 whereas the latest version is 16.0d2e1).