Developer reply by Paolo "Kaosmos"
Rated 4 out of 5 stars
Extremely useful and helpful! Almost 5 stars except that it is difficult to set options to launch links between Thunderbird and two Firefox profiles. Frequently get "Thunderbird|Firefox is already running" error. Sometimes links clicked from Thunderbird open in non-default Firefox profile.
(MOZ_NO_REMOTE and new_instance options need to be documented better).
Unfortunately there isn't a perfect solution for that problem, it's a limit of Firefox/Thunderbird core code. If you're not on Windows, the better option is to use "new_instance"; if you're on Windows, try to reset "MOZ_NO_REMOTE" at startup.
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