Based on your results, I tried installing the previous version of Firefox and the problem went away, so it is actually a problem on my end.
Turns out Firefox v119 uses "Rod" font, while Firefox v118 used "Consolas" and "Miriam Fixed" fonts, though I am not sure what changed - the browser's default font for "monospace" is "Miriam Fixed" in both versions. Either way, It's not a problem with MediaWiki.

In T347522#9204852, @IKhitron wrote:

Wait, @matmarex. People react to this and say there are more problems here, in Firefox only. I've asked to come here and to describe them.