Under Linux (Ubuntu and its derivatives for sure) there is only one way to amend this. Go to .mozilla-thunderbird, find the directory for your profile (something like rv7dwavi.default) and open the file prefs.js.

Then search for the strings  network.protocol-handler.app.http and network.protocol-handler.app.https.

Usually these directives are missing so you can safely add them anew as follows:

user_pref("network.protocol-handler.app.http", "google-chrome");

user_pref("network.protocol-handler.app.https", "google-chrome");

That's all and from now on Thunderbird will open all links in Google Chrome.

So if you think it is time to get rid of the old Firefox, the first thing to do is to make sure that it is set as your default browser. Of course, we assume you have installed it already

So, in terminal just type:

sudo update-alternatives --config x-www-browser

(I have found an alternative spelling for config with a single slash '-' which does not work for me )

Then just choose the number for /usr/bin/google-chrome, in my case it is 4.

The good part is that this default setting works not only with Gnome but also LXDE as in my case.