How to set default font in Mint
 

I am running Mint Debian edition, and I set the default fonts to a chinese type, I can see on Thunderbird using it but not in other applications, such as Whatsapp web with Firefox, and Google photo on Chrome using it but not with Firefox, where Signal not web based is using it I am quite confused, I wonder what does 'default' mean, and how to change that if no setting for individual applications ?

Firefox might not have the language pack installed for the language you chose. Search packages for alternate languages for firefox or you can tell firefox directly https://support.mozilla.org/en-US/kb...other-language