Different Tor Browser release channels (stable, alpha, nightly) share profiles on macOS

🐞 Bug Report

Reproduction steps

1. Launch Tor Browser Alpha (15.0a1)
2. Right-click on the tab strip and enable vertical tabs from the resulting context menu.
3. Close Tor Browser Alpha.
4. Launch Tor Browser stable (14.5.6)

Expected behaviour

The sidebar should not be enabled in Tor Browser stable, and an error informing me that "DuckDuckGo is no longer available as a default search engine" shouldn't appear.

Actual behaviour

The sidebar is enabled in Tor Browser stable in a broken state, and a message bar displays an error about DuckDuckGo:

Pressing the sidebar icon in the toolbar toggles expanding and collapsing the sidebar, but not dismissing it entirely.

Note: at no point were Tor Browser stable and Alpha open or running concurrently.

Bookkeeping

• Browser version: 14.5.6
• 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:

Troubleshooting

The following steps did not resolve the issue:

• Restarting Tor Browser stable.
• Creating a new profile in about:profiles and restarting the browser.
• Deleting the original profile after switching to the new one.

(I am slightly suspicious about this, so it would be worth someone double-checking the above.)

The following steps did resolve the issue:

• Closing Tor Browser stable, deleting the TorBrowser-Data file from the Application Support folder on macOS, and relaunching the browser.

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?

Introduced in #31457 (closed) (so awhile ago)