I just typed "stackoverflow.com" into the URL bar of Firefox (on Manjaro Linux), and saw this:

With experimentation, I found that Firefox is replacing all sorts of ligatures with (seemingly) random characters. Here's ff fi fl ffi ffl typed into the URL bar:

All the characters function normally: I still go to stackoverflow.com, but it just displays weirdly. What would have suddenly caused this to change?
Edit: this issue disappeared on reboot, but I'm still interested in learning why this occured.