I have my DPI in windows set to 175% because I like my text, icons, and apps extra large ( how it used to look back in the day on Windows 7 with lower resolution). Firefox and other browsers synchronize with windows to match your selected DPI. For example in my case wallpapers display at 175 % of their original size in my browser, so they'll look a lot worse then they actually are.
sorry if you already knew this, but i assume most people don't have their dpi set to 100% on 4k displays (I think 150% is the default for 4K), otherwise everything is tiny so I figured it's worth a mention. Type DPI in your windows search bar if you want to check what yours is