Firefox - Websites with custom fonts not displaying

This is a combination of a few things: TexLive is installing fonts in a not-so-standard location, and the webpage is requesting a font that Ubuntu's fontconfig package thinks is better fullfilled by one of the TexLive fonts rather than a normal font. Firefox is then told to use the TexLive font for rendering, but it's not allowed to reach it from inside its security sandbox.

The recommended workaround is to set

security.sandbox.content.read_path_whitelist to /usr/local/texlive/

in about:config, and restarting the browser.

This is fixed on a more fundamental level in Firefox 58 and later.

I too installed fonts-texgyre in my Debian 9 Stretch and the visualization of web pages has improved, but there are still some issues. Maybe we should install each font available in repositories, but it's clear that such a solution is not practical.

The only real solution would be to disable /etc/fonts/conf.d/09-texlive.conf.

If someone find another solution, it is welcome!