Firefox vs. Tor Browser fonts

Workstation: set to connect using the Tor before VPN method.
Browser: firefox-esr.
Problem: a certain language doesn’t get displayed properly on Firefox (instead of the letters, I see squares with numbers).

I looked at Change the System or Tor Browser Language - I think the KDE instructions are obsolete.

Searching elsewhere, I run sudo apt-cache search firefox-esr-l10n, found the package I need and installed it. It didn’t change a thing.
I also added the language as an alternative in the firefox preferences, no change.
I’ve used the dpkg-reconfigure locales to add it to the system, didn’t help.
I added it as an input language, as explained in Keyboard Layout - Kicksecure (Probably unrelated). I could then switch to this language and type, but didn’t help with my issue.

All that didn’t get firefox to display the fonts correctly.

Then I launched Tor-Browser, and I found out it’s ok there! it displays everything perfectly. Just not on firefox.

I then restored the workstation to its previous state, before all of the modifications above. None of them were necessary for Tor-Browser to do the trick. No modifications required. Works out of the box.

Looking for differences, I noticed Tor-Browser uses the Arimo font as default while firefox uses Dejavu serif.

Any idea how to make that work on firefox?