Steps to reproduce: 1. Have a "Firefox Nightly" app icon in your dock, but don't have it running. 2. Launch a locally-compiled Firefox build from the command line. This instance of Nightly appears as a new icon in your Dock and uses its own Firefox profile. 3. Click the "Firefox Nightly" Dock icon. Expected results: In the past, this would launch Firefox Nightly with your default profile, or (if you had checked the right checkbox in the profile manager) it would show the profile selection window. Actual results: Now the Dock icon bounces once and then nothing happens. The new behavior is somewhat similar to the behavior we have on Windows + Linux, but IIRC on those platforms we will at least focus the existing instance and open a new window with it. Just doing nothing is very confusing.

It would be good to narrow down a regression range. Did this happen in 65 or is it new behavior in 66?

Fixed by the backout of bug 469990 (cset 590bdef6b312) which will be merged to beta shortly.

The backout has been landed on Beta and will be included in the 65.0b5 release. https://hg.mozilla.org/releases/mozilla-beta/rev/978402f68f3c