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Once you install both Firefox (stable channel) and Firefox Nightly, you'll notice that, by default, you cannot have both opened at the same time. In order to achieve that, one has to first create a second profile and link it to Nightly somehow, i.e. make Nightly use that second profile.

Mozilla's Profile Manager offers some options and I've been trying to set it up correctly, but to no avail:

My tweet w/ screenshot:

Could you please help me configure this Profile Manager correctly?

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It could be easier to set this (multiple profiles) up using the builtin Profile Manager. Please see this. Also see Builtin PM. Once a second profile is created (e.g. name nite), you could create a Nightly shortcut on the desktop and on the shortcut's Target property add a space after firefox.exe", followed by -p nite -no-remote – vWil Nov 24 '13 at 14:19
@vWil Woo hoo! It worked. Finally! – Šime Vidas Nov 24 '13 at 23:28
Thanks for the comprehensive update :) Also, if there's a space in the name use quotes e.g. -p "Fx nite". If Firefox is to be started second, the same needs to be done to its shortcut but with the other name for -p. One thing to note is that if started via -no-remote (Firefox or Nightly), then links from external programs may not work if it is the default browser, unless Firefox or Nightly (whichever is the default browser) is first started via an external program/link. – vWil Nov 25 '13 at 14:17
Multiple profiles, Command line options – vWil Nov 25 '13 at 17:41

Thanks to @vWil's advice I was able to finally make it work. Here is my step-by-step guide. Note: I'm performing this on Windows 8.1. I'm not sure if the process is different on OS X.

Step 1: Create a second profile

Note: Firefox must be closed while performing this step.

Press Win + R to open Windows’s “Run” dialog, then type "firefox.exe -p" into the input field and press Enter.

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This should open a "Choose User Profile" dialog:

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Click on the "Create Profile" button and follow the steps. Use a simple name for this profile. I've chosen "nite":

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Step 2: Install Firefox Nightly

The download page is here.

Step 3: Set command line flags

Locate the launch icon for Firefox Nightly, right click on it and go to Properties:

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In the Properties dialog, add -p nite -no-remote to the Target field:

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If you've set a different name for the second profile, use that name instead of nite, e.g. -p foo -no-remote.

That's it

Open Firefox, then open Firefox Nightly using this custom launch icon. Both should be able to run at the same time:

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Please let me know via comments if this process didn't work for you.

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awesome! Works on win7 – David 天宇 Wong Mar 3 '15 at 0:37
I've created a bug report: Create "Nightly" profile to make it easy to run Nightly alongside stable/normal Firefox – nachtigall Apr 18 at 11:46

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