meek profile error after browser restarts (e.g., after update or add-on installation)

While testing an in-browser update of TB 4.0a2 to TB 4.0a3 with meek enabled, the meek browser profile seems to be damaged after the update. brade and I observed this on both Mac OS and Windows 7. Steps to reproduce:

1. Configure TB 4.0-alpha-2 with the meek-amazon transport.

2. Use the in-browser updater to update to 4.0-alpha-3 (for testing, we are hosting the XML update manifest and MAR files on our own server and setting app.update.url.override to point to them).

3. After restart, the Tor Status window will make no progress and an error alert that reads "Profile Missing. Your Firefox profile cannot be loaded. It may be missing or inaccessible."

Strangely, restarting the browser a second time seems to fix the problem.