Here's what I finally figured out:
In Thunderbird go to "Edit --> Preferences --> Advanced" in the Thunderbird menus and click on the "Config Editor" button.
Search for the following three entries:
network.protocol-handler.warn-external.http
network.protocol-handler.warn-external.https
network.protocol-handler.warn-external.ftp
Set the value of each of these three entries to true (you can do this by double-clicking on each entry)
Right click on white space in the Config Editor window and click on "New --> String".
Enter the preference name "network.protocol-handler.app.http", click on "OK" and then enter string value "/usr/bin/google-chrome".
Click "OK"
Repeat to create strings "network.protocol-handler.app.https" and "network.protocol-handler.app.ftp", both with the same value "/usr/bin/google-chrome"
Close the "about
:config" window and click "OK" on the "Thunderbird Preferences" window.
Having completed these steps, the next time you click on an http, https or ftp link in Thunderbird, you will be presented with the "Launch Application" window. Press the "Choose" button on this window to select your browser of choice. This is a one-time task; having selected the browser once, links of the same type will now always be opened with the selected browser.
(Credit: I've quoted pits and pieces from a number of sources, too many to remember, and added some trial and error of my own to come up with this solution. Portions of the text above is based on
Default browser - MozillaZine Knowledge Base)
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