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?