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Firefox is my browser of choice, and I'm struggling to keep a second Firefox in my system to use for testing purposes (cucumber, selenium and geckodriver, if you're curious). The second Firefox is an older one (45) that is used only as headless browser to run acceptance tests. Distribution is Manjaro Linux:

ngw@blackstar ~ uname -a
Linux blackstar 4.9.20-1-MANJARO #1 SMP PREEMPT Fri Mar 31 17:49:34 UTC 2017 x86_64 GNU/Linux

I've tried to download the Firefox I need, unpack it into /opt and ln -s it in /usr/local/bin with a different name (firefox45):

ngw@blackstar ~ ls -l /usr/bin/firefox45
lrwxrwxrwx 1 root root 20 May  6 17:39 /usr/bin/firefox45 -> /opt/firefox/firefox
ngw@blackstar ~ firefox45 -v

Mozilla Firefox 45.0

Unfortunately for reasons I can't explain that executable starts my real, up to date Firefox (53.0), I double checked with about: inside the browser.

What's the easiest possible way to solve this problem?

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By default, if an instance of Firefox is already running, it won’t start a new one unless you specify --new-instance.

If you want to run two instances of Firefox simultaneously, with different versions of Firefox and for different purposes, you should use different profiles: your default profile, presumably with your default Firefox, and another profile for your test Firefox.

To set that up, quit Firefox altogether, then run

firefox45 --ProfileManager

and create a new profile (e.g. “acceptance”).

Then you can run Firefox 45 with

firefox45 -P acceptance

and it will use that profile, and start a new instance instead of using your existing default Firefox.

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The executable firefox is not the actual executable. It is a shell script:

#!/bin/sh
exec /usr/lib/firefox/firefox "$@"

If you unpacked Firefox into /opt, you probably have /opt/…/lib/firefox/firefox, which is the real Firefox 45.

As @StephenKitt mentions, you may still want to give the --new-instance flag, and you definitely want a second profile.

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