Firefox esr can't be added fonts

I installed firefox esr on my ubuntu 20.04.
I found I can’t add custom font into firefox esr.

Is there a way to add fonts into my firefox esr?
I would like to add Luxend from Google Fonts.
I have installed this my machine already.

Hi. I moved this topic to #cafe since it does not seem to be about snaps. If this is about Ubuntu in general might be better suited for this, but if it is more of a support question, is the place to go.