My Fonts Won't Show Up

Good evening, dear reader. I am an upcoming web developer. When I load my HTML file that has Arial chosen as a font type, I see Courier instead. I’m using Firefox Browser 79.0 (32-bit). In the About Mozilla Firefox bar or panel, I see:

“ Mozilla Firefox for Ubuntu
canonical - 1.0”. What could I be doing wrong? I am sorry if this question is in the wrong category. I don’t know where else to post it.