I use the Windows registry FontSubstitutes (Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\FontSubstitutes) to replace system fonts like Segoe UI with other fonts. It takes a lot of work to get Windows itself to respect these options, to the extent that I have to make it believe Segoe UI isn't present by modifying a different registry location (Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts). However, Firefox does not respect the FontSubstitutes settings. Based on a previous post here, this is something it did prior to Firefox 89, but has stopped doing. An ideal solution would be a way for Firefox to correctly read my FontSubstitutes settings. An acceptable workaround would be a way to provide Segoe UI for Firefox to use while still preventing its use in other applications. (I know that, for instance, Adobe Photoshop allows users to install fonts to Photoshop itself, bypassing the system fonts folder entirely.)