I've just been playing around with https://browserleaks.com/fonts . It seems no web browser provides adequate protection for this method of fingerprinting -- in both brave and librewolf the tool detects rather unique fonts that I have installed on my system, such as "IBM Plex" and "UD Digi Kyokasho" -- almost certainly a unique fingerprint. Tor browser does slightly better as it does not divulge these "weird" fonts. However, it still reveals that the google Noto fonts are installed, which is by far not universal -- on a different machine, where no Noto fonts are installed, the tool does not report them.
For extra context: I've tested under Linux with native tor browser and flatpak'd Brave and Librewolf.
What can we do to protect ourselves from this method of fingerprinting? And why are all of these privacy-focused browsers vulnerable to it? Is work being done to mitigate this?
I wonder if running it in a container such as flatpak would help.
Flatpak is not a container and should not be thought of as such for security/privacy purposes:
https://flatpak.org/faq/#Is_Flatpak_a_container_technology_
It can provide container-like functions if specifically configured for that, but that's not normal and it shouldn't be relied on as a security barrier.
I would not count on it, since it's required for proper theme integration. A quick search confirms my suspicion: some font direcories are mapped.
I quite like the idea though, sort of a lite qubes or unmodified VM for all Firefox Flatpak users could be nice.
In a perfect world, it would be nice to have a checkbox per app where I can select whether it should share anything with the system libraries.
Not sure whether it can fix the font problem, but in general Flatseal allows you to customise permissions for installed flatpaks.
https://flathub.org/apps/com.github.tchx84.Flatseal
I'm running Brave and Librewolf from flatpak. Nope, it doesn't help, at least with default sandbox settings.