You have three options.
- Use a portable version of firefox.
You can check the link for details: Using portable firefox and fixing flash
- Use Virtual machine and use firefox therein.
- 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 ->
(Your newer version of firefox will not be affected, and it has
firefox --no-remote -P "default" as command.)