This is a weird issue. There seems to some security thing in Firefox that's preventing the custom fonts from loading. The network tab gives the error Request for font "Cabin" blocked at visibility level 2 (requires 3). Works fine when the session isn't private. Chrome doesn't have this issue at all.
Going to have to look into this later. This SO issue might help: https://stackoverflow.com/questions/75978659/what-does-the-message-request-for-font-noto-sans-blocked-at-visibility-level