How do I install a font on my iPad^©®™? I clicked on download but i couldn’t install it anywhere

Suggest you try searching "how to install font on iPad^©®™".
You can get articles (like this https://www.howtogeek.com/350438/how-to-install-fonts-on-an-ipad-or-iphone/),
official help (like this https://support.apple.com/en-au/guide/ipad/ipad952422ba/ipados),
and even videos (like this https://www.youtube.com/watch?v=T3rsyx601ag).

how do i install fonts on firefox? its doesn't do anything

you want to change the UI of Firefox ?