Bug 1531719 Comment 2 Edit History

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With bug 1518639 landed, this issue is fixed & verified. Basically, now it is possible to have release(or any other different install) opened and just open Nightly, even if it wasn't opened before to generate it's dedicated profile.

Also, the --new-instance parameter is not needed for this particular scenario. Just calling ~/Documents/release/firefox and ~/Documents/nightly/firefox will automatically trigger the two separate instances.
The only note I would add here, in light of using the --new-instance parameter is that you cannot run the same install with two profiles without using the profile manager((e.g. calling ~/Documents/nightly/firefox --new-instance and ~/Documents/nightly/firefox), in which case you will still hit the reported error message, but that would be the expected result.
With bug 1518639 landed, this issue is fixed & verified. Basically, now it is possible to have release(or any other different install) opened and just open Nightly, even if it wasn't opened before to generate it's dedicated profile.

Also, the --new-instance parameter is not needed for this particular scenario. Just calling ~/Documents/release/firefox and ~/Documents/nightly/firefox will automatically trigger the two separate instances.
The only note I would add here, in light of using the --new-instance parameter is that you cannot run the same install with two profiles without using the profile manager((e.g. calling ~/Documents/nightly/firefox --new-instance and ~/Documents/nightly/firefox --new-instance), in which case you will still hit the reported error message, but that would be the expected result.
To correctly start two nightly instances with the same installation path would be to:
~/Documents/nightly/firefox -p (select first profile to use)
~/Documents/nightly/firefox -p (select second profile to use)

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