When Thunderbird starts it ignores already existing registry entries. That means, if you have had deleted an extension via UI and you did not change the registry entry (deleting or modifying) afterwards TB will not install this extension again.
In other words: If you have once installed an extension via registry and you delete it manually via UI keeping the depending registry entry, this extension will not be re-installed automatically. You have to delete/modify this registry entry.
Full pathes to files on shares are O.K.
Your registry entries:
Software\Policies\Mozilla\Thunderbird\Extensions\Install\1 = "//yourserver/yourcerts/your.xpi" Software\Policies\Mozilla\Thunderbird\Extensions\Uninstall\1 = "email@example.com" Software\Policies\Mozilla\Thunderbird\Extensions\Locked\1 = "firstname.lastname@example.org"
These email addresses are the IDs of these extensions, you will find them here:
Start TB, go to the AddOn manager, go to options (the gear symbol), select Debugging AddOns.
Locked means, no user can delete this AddOn manually.
Always check: https://github.com/drlellinger/thunderbird-policies for details. Credits to this guy, he did a lot of work. TB is very poor documented.