Tor Browser 15.0a1 starts Firefox

🐞 Bug Report

Reproduction steps

On a pretty fresh install of Debian 13 (Trixie):

1. Download tor-browser-linux-x86_64-15.0a1.tar.xz.
2. Verify the GnuPG signature.
3. Unpack with tar xvf tor-browser-linux-x86_64-15.0a1.tar.xz.
4. Execute tor-browser/Browser/start-tor-browser.
5. Firefox starts instead of Tor Browser.

I discussed this with @ebanam over email who suggested to report as a bug.

Following his advice, I also tried ./start-tor-browser.desktop --register-app and got the same behavior:

Screencast_From_2025-08-20_09-58-57

Following the same steps on the same machine with tor-browser-linux-x86_64-14.5.5.tar.xz works fine.

Attaching the output of running with --verbose.

• 14.5.5.out
• 15.0a1.out

Bookkeeping

• Browser version: 15.0a1
• 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: Debian 13 (Trixie)

Browser UI language

English