fontconfig that survives Firefox snap updates?

Is there some way to have a custom fontconfig for the Snap-distributed Firefox that survives Firefox updates?

Background

I have a custom fontconfig at ~/.config/fontconfig/fonts.conf which worked fine when Firefox was installed via apt. With 21.10 distributing Firefox via Snap, it doesn't register my custom config unless it's in ~/snap/firefox/<somenumber>/.config/fontconfig/fonts.conf.

The problem is <somenumber> seems to change on update so I'm going to have to apply this over and over potentially. I tried dropping the file in ~/snap/common/.config/fontconfig/fonts.conf and even /etc/fonts/conf.d/99-custom-fonts.conf but those don't seem to be loaded.

Solution 1:

As regards ~/snap/firefox/<somenumber>/... not surviving snap updates, I'm not aware of any solution. This comment indicates that the issue is identified (even if that thread is about the Chromium snap).

But putting it in /etc/fonts/conf.d will hopefully work soon. I brought up the topic in this Discourse comment, and there is already a pull request which is said to be the first step in a fix.

Edit:

Maybe I stumbled upon a solution after all: Simply put user specific conf files in conf.d/ instead of editing fonts.conf. Please see this comment.