This wasn’t the case until my last restart. The font is set to Clear Sans in Firefoxs settings. I have also noticed this in certain elements of an appImage I run called Caprine, which is an electron app which brings Facebook Messenger to the desktop.
I normally use Gnome however I recently installed KDE Plasma. This had not been an issue after installing and restarting that, so I assume something more recent has caused the issue, such as an update.
Edit 1: I have also just discovered that my ffmpeg (installed from this guide) no longer seems to function in Firefox. rebuilding and reinstalling ffmpeg seemed to fix this.
Edit 2: after restarting Firefox, I have just been greeted by a fresh update page, informing me I am running the latest version.