See https://support.mozilla.org/en-US/kb/Hyperlinks-in-Messages-Not-Working

I followed everything on the link given above, on every page, except for the Standard Diagnostic page. It looks old to me and besides, I could not find the "XUL.mfl" file anywhere. Did find xul.dll. I did points 1 to 3 on the diagnostic page. Sorry, but I'm thinking even if I followed through the issue would not be solved as the page quotes Win98 / WinXP, and never mentions Win7 or beyond.

This is really annoying. It used to work ok and now this. Any link to open a new email does work though.

Interesting, I did not have that link to standard diagnostics when I first wrote that article some 5 years ago. You are correct it is dated, and really about as useless as it can be

However I digress.

In the config editor expand your criteria by only including network.protocol-handler in the search. Anything that is bold right click and select reset.

from the help menu select troubleshooting information. Click the show button for the profile folder Close Thunderbird. Delete the file mimetypes.rdf. Restart Thunderbird.

I do not expect either of these things to fix the issue as my gut feeling is you have anti virus issues. These pop up with blocked links after just about every new Thunderbird release. Norton's products get a lot of issues, but any that pretend to scan for spam, scams or have a firewall are regular problem children as is other security software such as that used by banks and the US military.