Navigation panel icons are broken if custom fonts are not allowed in Firefox

In Firefox 47, without any addons or tweaks, when you de-select “allow pages to choose their own fonts”, the icons on the sidebar are broken.

Some non-sidebar icons, like Github and Bitbucket ones, seems to be unaffected, even when they are made using fonts too.

There is a related discussion at and it’s marked as fixed - as far as I understand, the icons that are done with characters from Unicode Private Use Area are all right:

Firefox default fonts are used but webfonts are allowed when
they contain characters not covered by the users default fonts (like icons)
(Comment 143 in that discussion)

I was not allowed to include more images in the post as a new user, so, here is where it’s set up in firefox:

This dialog lives under Preferences -> Content tab -> Fonts section -> Advanced button.

Icons in help popup are working all right too.

Linux and OS X icons are unaffected as well.

Project Settings icon seems to be not working too.

Icon for closing notification is affected too.

Builds summary page has some affected icons too:

SSH key management section has yet another affected icon.

I’m seeing this with Firefox 48 on Linux too. It’s quite annoying.