Browser UI uses the wrong font (appears like a default serif font)

🐞 Bug Report

Tor Browser Alpha seems to not load the correct font for the browser UI. Specifically it is some serif front rather than a sans-serif font. The font is still usable, but it's clear that the correct font is not being used. This is not an issue on the latest stable 14.5.8 release, only on the latest alpha 15.0a4.

Notably if I launch the browser with wayland enabled (GDK_BACKEND=wayland ./start-tor-browser.desktop), the correct font loads.

Reproduction steps

Download tbb alpha, extract, run start-tor-browser.desktop.

Expected behaviour

The UI should have a sans-serif font. Unless the browser font changed, I'd expect it to be the same as Firefox and Tor Browser 14.5.8.

Screenshot from 14.5.8:

Actual behaviour

Screenshots from 15.0a4:

Bookkeeping

• Browser version: 15.0a4
• Browser channel:
  • Release
  • Alpha
  • Nightly
• Distribution method:
  • Installer/archive from torproject.org
  • tor-browser-launcher
  • homebrew
  • other (please specify):
• Operating System:
  • Windows
  • macOS
  • Linux
  • Android
  • Tails
  • Other (please specify):
• Operating System Version: Fedora 42 + sway

Browser UI language

en-US

Have you modified any of the settings in about:preferences or about:config? If yes, which ones?

No.

Do you have any extra extensions installed?

No.

Troubleshooting

Does this bug occur in a fresh installation?

Yes.

Is this bug new? If it is a regression, in which version of the browser did this bug first appear?

Yes. Unsure which version, but likely after the ESR update? It's fine on 14.5.8 stable. I can narrow it down more if needed.

Does this bug occur in the Alpha release channel?

Yes.

Does this bug occur in Firefox ESR (Desktop only)?

Can test if needed.

Does this bug occur in Firefox Rapid Release?

Can test if needed.

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