+++ This bug was initially created as a clone of Bug #668474 +++ If would appear that webcal links are not working. Both links in Firefox and Thunderbird mail prompt a select application dialog, which does not work as it has no way to use the command line parameter. Perhaps for windows at least we can register the webcal URL in the registry as a part of the system integration providing Thunderbird with the necessary parameters to make this work for all links coming into the machine. SUMO report on Lightening 126.96.36.199 with Thunderbird 38.4.0 on Ubuntu 15.10, I can confirm behaviour on Windows 7 with nightly and lightning 4.8a1 HKEY_CLASSES_ROOT\webcal\shell\open\command
Webcal is an unofficial URI scheme invented by Apple. Simply change your URL from "webcal://..." or "webcals://..." scheme to official "http://..." or "https://..." scheme that is known and supported by Thunderbird to access the calendar.
I know all that, but Bug 668474 did apparently include support. So this is a regression. On top of that, official or not it is in use. Perhaps by a small number of folk, but given the presence of Apple devices in the market quite possible quite a large number as apple devices will be emailing these links I would guess. This is also something of an Outlook parity issue as Outlook makes the necessary registry entries to make opening the links transparent.
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