LibreOffice Help (F1) opens a thunderbird new message and not a web browser - how to I fix this?

Running Linux Mint 20.1 with LibreOffice 6.4.7.2. When I try to access the LibreOffice Help F1 browser link in any of the apps (Writer, Calc, etc) - it opens a new message page in my default email client Thunderbird. Why is this and how do I fix this to point Help to my default browser Firefox?

Check if on a terminal command line xdg-open https://help.libreoffice.org/ opens in a web browser or in Thunderbird as well. If in Thunderbird then your system is malconfigured and needs further checking.

Thanks erAck: The output from your suggestion is the following before it fires up Thunderbird.
(gio open:4419): GLib-GIO-WARNING **: 10:32:33.465: /home/colin/.local/share/applications/defaults.list contains a [Added Associations] group, but it is not permitted here. Only the non-desktop-specific mimeapps.list file may add or remove associations.

Searching the defaults.list file for “libreoffice” or “help” found no occurrences. An Internet search on the full error above gave some answers but they were all particular and not generic to the overall cause. Where can I go from here?

Check what xdg-settings get default-web-browser gives, it should be some userapp-Firefox-*.desktop that then could be found as ~/.local/share/applications/userapp-Firefox-*.desktop file and should contain a Exec=/usr/bin/firefox %u line. All that seems to be not the case for you, go to your System Settings and in Default Applications for Web choose Firefox.

Thanks erAck: The output from your suggestion is the following before it fires up Thunderbird.
(gio open:4419): GLib-GIO-WARNING **: 10:32:33.465: /home/colin/.local/share/applications/defaults.list contains a [Added Associations] group, but it is not permitted here. Only the non-desktop-specific mimeapps.list file may add or remove associations.

Searching the defaults.list file for “libreoffice” or “help” found no occurrences. An Internet search on the full error above gave some answers but they were all particular and not generic to the overall cause. Where can I go from here?

The problem, as outlined in my final comment above, is that you have to make Firefox your default browser before you can change the Preferred Web app in Linux Mint from thunderbird to Firefox. Only then does LibreOffice Help go to a Firefox web page rather than Thunderbird. I’m guessing that it doesn’t matter which browser you use - so long as it’s set up as your default browser. Unfortunately my final comment which detailed what I had done to resolve this somehow got lost - but the answer to my original question is contained above.

That’s an odd handling in Linux Mint ¯\(ツ)

You may want to report a bug at their bug tracker that having Thunderbird as default browser if no browser is set as default is utter nonsense. (well, in slightly less strong language maybe…).