https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_moving-a-profile isn't outdated afaik

Wayne, thank you very much.

Can I presume from your response that I can move the profile (which is the vast majority of the space required: >8GB), but not the TB program files (~250MB)?

James' response on the related question for Firefox can be found here: https://support.mozilla.org/en-US/questions/1449842 , which appears to allow me to do that by using the MSI.

I note that there is a TB MSI installer here: https://www.thunderbird.net/en-US/download/ . Would that allow me the same capability? If so, I repeat the question on the other thread: will the automatic update function put the updates in the correct place?

Just to jump in here, the install offers the option of installing on any drive. And auto updates are applied there. If thunderbird is installed on C drive, the easy way to create profile on another drive is to define a folder on the other drive, then click help>troubleshootinginformation, scroll to 'profiles', click 'about:profile', then click the 'create profile' button, click next, enter a name, such as 'PROFILE', then click the 'choose' button to locate the folder you created on the other drive and make that your default. You can also move the profile to another drive from the drive that Thunderbird is on. I'm not sure which approach you want.

Thanks again.

At this point, I tried and am happy with the portable version supplied with PortableApps.com -> "A:\PortableApps\ThunderbirdPortable\ThunderbirdPortable.exe" Performance is quite acceptable (only slightly slower popping on; otherwise indistinguishable).

Porting my prio profile into the new profile was event free and works great. I put the TB logo on my TaskBar and all is well.

After I get everything else running, If there is space to spare, I may transfer the APP to C: and keep the profile with its many Gig of email history on A:, as suggested -- just to see if it pops quicker (I doubt it will, as I expect that it is all the profile data slowing it down).

I could also end up putting on C: if I end up having some glitches like I had with the portable Firefox version from PortableApps [other programs struggled to open Firefox as the default specified in MS default-apps on the A: drive.]