Thunderbird ignoring ReadNotify extension

Prior to updating my laptop recently (which included W7 to W10) ReadNotify worked well. Since the upgrade however Thunderbird refuses to add the suffix to my outgoing emails. Contact with ReadNotify has narrowed this down to Thunderbird seeming to ignore the presence of ReadNotify on my computer. Despite reinstalling Thunderbird at least twice the prompt to allow the ReadNotify add-on fails to appear and the list of add-ons in Thunderbird does not include ReadNotify or RN5 despite the software showing it as installed and active on my computer. Tracking works if I manually add to outgoing email addresses but it won’t add automatically as it should. Has anybody got any suggestions?

if this is an extension, it probably hasn’t been updated by ReadNotify to work with the current version of Thunderbird.

Add-on support: Add-ons are only supported if add-on authors have adapted them

REF: Thunderbird 68.0 Release Notes

That is most likely why Thunderbird is ignoring the presence of it.