"Firefox is already running" when you select meek after bootstrapping

1.Let Tor Browser bootstrap without any pluggable transports. 2. Open Network Settings and choose meek.

An alert appears: Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system. After that you can't browse. But closing the browser and allowing it to bootstrap from scratch again (with meek) works.

Tested on 3.6.3-meek-1 and on a build of the 4.0-alpha-1 branch.