Not sure if this is new with Fx57 or if it's a problem with the Dev edition, but this only happens since I switched from 56 to Dev. When I click a link in Adobe Reader or Thunderbird it will open that in a new window of my already running Fx Dev instance. I'm pretty darn sure this hasn't been the case previously. Except for allowing legacy extensions, I didn't change anything in about:config. I don't suspect TabGroups extension to have anything to do with that and my other installed extensions are all Webext, which doesn't even have the capability of changing such a critical component of the browser. browser.link.open_newwindow is set to the default of 3, but as far as I could find out this should only affect internal links originating inside the browser. browser.link.open_newwindow.override.external is set to the default of -1, which isn't even a valid value according to documentation, but to be honest I'm not sure what the meaning of this setting is. I could only find other support.mozilla.org entries referring to that setting, but no documentation or wiki about it.