nowadays, here's the new way to still "make it happen":
Navigate to about
:config, confirming the risk by clicking the button.
Type webcal, filtering the list to items of type gecko.handlerService.schemes.webcal.[...].
Click the Edit button at the right of your chosen [...].name item (since 30 Boxes was abandoned with Firefox 67, I opted for [...].0.name)
Type in your desired name, e. g. Google Calendar and click Save.
Click the Edit button of your corresponding [...].uriTemplate item, i. e. the one just below your edited one.
Paste the URL
https://calendar.google.com/calendar/r?cid=%s and click Save.
Go to Firefox's options for associated applications by consecutively clicking ≡, ⚙ Options, ⚙ General menu options and then scrolling down to the Applications paragraph.
Search the list for webcal by either typing it into the search box just above the list or via scrolling.
Click the drop-down at the entry's right half and select Use Google Calendar (or whatever name you chose at instruction #4).
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