Not the webpage font, but url and tabs font, this is more of a bug report as my firefox installation has this hidious eye sore of a font and does not use Segoe UI as a default font for some reason, either that should be the default font or at least have the option to change browser font, this is currantly changable by creating a file named "chrome" in firefox's profile folder inside its release version and .css file called userChrome.css and then adding a font-family: Segoe UI !important; script - but it should be changable in settings as firefox looks bad otherwise or at least make the default font prettier on desktop
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We may be able to add a preference to make this process easier. Feel free to track this report and stay tuned here for any updates.