Firefox selects wrong typeface weight
Recently Firefox (both regular and dev edition) started showing monospace fonts in SF Mono Heavy variant. I use macOS 11.4 and Firefox Developer 90.0b6 but it happens with Firefox 89.0 as well, even in Troubleshooting Mode.
Below is a screenshot of a GitHub page. The same page rendered in Safari or Chrome on the same device show normal variant of SF Mono. Notice that the code on the page is rendered in heavy weight.
Running inspector mode shows that Firefox selects SF Mono Heavy even though regular variant is clearly present in the system.
I have tried disabling hardware acceleration and webgl. Are there any other ways to troubleshoot and fix the bug?