Firefox is using a monospace font - and other issues

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.

Clear Sans is not monospaced, so do you mean you Firefox is using some other monospaced font instead of Clear Sans?

Since Firefox can set different fonts sets for each locale, can you check whether some locales set monospaced fonts as their defaults?

I mean that despite Clear Sans being the set font, there is another monospace font over all UI elements. I have also noticed this is the case across some Plasma components, but not all.