Running multiple Firefox versions/profiles on the same computer

[Information Source]

To start with we are talking about “running” and not just “having” two different versions. Furthermore, we can have two different profiles of the same version wherein you can have useful but resource-heavy add ons in one profile and a lighter other profile. Here is a good link that can help us achieve it. For the record, I am just summarising what is there on the site:

1. Get FF Profile Manager: Close all FF instances. Start FF from command line using the following commands:

For Windows: <Firefox installation folder>/firefox.exe -ProfileManager

For Linux: <Firefox installation folder>/firefox -ProfileManager

2. Create profile: Once you start FF with the command above you will get a Profile Manager dialog. Create a new profile, assuming you are going to use the default one as the other profile.

3. Create shortcuts: for the new profile as well as for the default one.

For Windows: Right click on desktop and Create New Shortcut. Target field should look something like:

<FF installation folder>\firefox.exe -P  <profile name> -no-remote

For Linux: Create a new command and enter the following:

firefox -P  <profile name> -no-remote

Create one more shortcut  – this time the profile name being “Default”. 

Reproducing verbatim from the source link

What the -no-remote flag (vaguely) tells Firefox to do is start a completely new and segregated Firefox process

My two cents: A beautiful fact is you can use this approach to have FF2.x and FF3.x running on the same computer simultaneously. Just ensure your shortcuts point to those versions respectively.