Environment: Windows 10 with an existing default install of Mozilla Thunderbird ("C:\Program Files\Mozilla Thunderbird")
Install desired second version (e.g., 60.9.1) to a new directory (e.g., "C:\Program Files\Mozilla Thunderbird 60") by selecting "Custom" as setup type
Once installation completes, uncheck "Launch Mozilla Thunderbird now" and click Finish
Edit Thunderbird 60's Desktop shortcut, appending a path to a new data directory (e.g., -profile "C:\Users\user\Documents\TB60DATA") to the Target field, which should then read "C:\Program Files\Mozilla Thunderbird 60\thunderbird.exe" -profile "C:\Users\user\Documents\TB60DATA"
On first launch, close the "Set Up an Existing Email Account" and "System Integration" windows
Right click the title bar then click "Menu Bar" > File > Offline > Work Offline
Click Tools > Options > Advanced > Update > check "Never check for updates" > OK
At this point, you can setup an account, add extensions, etc.
In this example, we'll add Kaosmos' ImportExportTools and his updated version of Torge Kummerow's Email Address Crawler, both of which are compatible with Thunderbird 60.* (but no higher; see Christopher Leidigh's ImportExportTools NG for more recent versions):
Download XPI files
Set extensions.strictCompatibility to false in Config Editor (Tools > Options > Advanced > General > Config Editor) to enable Kaosmos' updated Email Address Crawler
Install XPI files (Tools > Add-ons > Extensions > drag XPI files into window)
/windows | Jun 07, 2020