Bluish Coder

Normally when you run Firefox it looks for an existing Firefox process that is running, and starts a new window running within that process.

Sometimes you want to start a new process so if it crashes it doesn't bring the other running process down. Or if you want to run a different Firefox version you want to be able to start it in a separate process. Otherwise a new window for the old version will open.

One way of doing this is to set the environment variable MOZ_NO_REMOTE to equal '1' and then run Firefox. This will start Firefox in a new process. For example, under Windows:

set MOZ_NO_REMOTE=1
firefox -ProfileManager

Or Linux running bash:

export MOZ_NO_REMOTE=1
firefox -ProfileManager

I use '-ProfileManager' to enable creating or selecting a new profile for the new running process so it doesn't conflict with the one used by any existing process. I also often create/use/delete temporary profiles for testing purposes.