I've just run a fourth workstation and Thunderbird successfully installed an update to 60.5.0. Has this update broken the two other copies?

It looks to be a permission issue. If I run TB as administrator it does appear to work on these machines. The problem being I don't want all users to have full admin rights on their machines as this is opens up other security issues.

Ok, so the crashing is still happening. It appears to relate to the 'local folders' which are actually on a network drive. This is done because we actually share the local folders, something we've done successfully for about 15 years. Now however on these two workstations certain actions on the local folders cause the crash. TB will then continually restart & crash until told to stop. Re-starting as administrator will break the cycle as the local folders are not available, allowing the user to set their inbox as the active folder and they can then restart as a normal user and the shared local folders will again be available to them.

So I'm still looking for a long term solution to this which wasn't a problem until v60.5

Based on what you have said. A couple of questions but nothing helpful probably.

Are the permissions for the network share set using a UNC or a drive letter? Changes in the release notes mention changes from drive letter to UNC paths, this might be related? Just a guess.

It is a mapped network drive, so yes linked to the drive letter. This might be part of it, but I've not changed the settings on the other workstations and they continue to work as they have always done.

Indeed for one user I've not changed their network permissions and it has not been affected (so far at least).