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This is a spinoff of ticket #16014 (moved). Georg noticed that after he updated Tor Browser 4.5a5 to 5.0a1, he saw a "Checking Compatibility of Add-ons" window each time he started the browser. Kathy and I debugged this and found that this window is coming from the meek helper browser. It shows up repeatedly because the prefs.js file is not being written to the profile (presumably because the meek browser is killed and does not exit in a clean manner).

One way to fix this is to add code to the meek HTTP helper extension that flushes the browser prefs. to disk before it enters the blocking event loop. There may be a better solution, but this seems to solve the problem.