How to install an old version of Firefox side-by-side to default version without conflicts?

You have three options.

1. Use a portable version of firefox.

You can check the link for details: Using portable firefox and fixing flash

2. Use Virtual machine and use firefox therein.
3. Install two versions of firefox. (Probably this is what you are looking for.)

Download the older version of firefox from the archive: Mozilla.org_ftp_site

Unzip to a path.

Next open terminal and use the following command to add a new firefox user.

firefox --no-remote -P

Create a new user, take note of the user-name.

Next, create a launcher for the 'firefox' you just unzip. Go inside the unzipped folder , there you will find a firefox file (probably binary) which launches the program. When creating the launcher, update the command as:

/path/to/unzipped/firefox --no-remote -P "<user-name>"

Creating launcher in Ubuntu

And now, when you will use this launcher then older firefox (which you want to use for testing) will open. Important - check the build version to verify.

In the url bar -> about:

(Your newer version of firefox will not be affected, and it has firefox --no-remote -P "default" as command.)

You should create a new Firefox user profile
and install by the -no-remote command line
Please visit: https://support.mozilla.org/en-US/questions/974208