To choose browser of your choice to open links go to
Edit > Preferences > Advanced > General > Config Editor
and search for network.protocol-handler.warn-external.

If the following three are all set to false (default), turn them to true, and Thunderbird will ask you when clicking on links which application to use (remember to also check “Remember my choice for .. links”).


network.protocol-handler.warn-external.ftp
network.protocol-handler.warn-external.http
network.protocol-handler.warn-external.https
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