Update 2018-10-17: The plugin I discuss below no longer works with current versions of Thunderbird. Fortunately, there is another that provides the same functionality and is still compatible. See my followup post for details.
Quite a while ago my corporate calendar got switched from an internally hosted system to THE CLOUD!!!!
Ahem. Sorry, my buzzword detector went off and I flinched and hit caps lock. Don't ask me how that resulted in all of the !'s. ;-)
Anyway, the transition wasn't immediate so I kept my old calendar around for a while. It was integrated into Thunderbird via the Lightning extension, and everything was pretty happy. When I got my Google calendar, rather than add it to Lightning and end up with a bunch of duplicate events, I simply opened it in a browser window and used that for a while. This worked fine for most things, but for some reason I don't get notifications from the browser-based calendar. This became more of a problem as more of my events migrated to Google calendar. Recently I've started leading a meeting on a fairly regular basis, which means I kind of need to be there, and there's no one to ping me that I missed the notification for it. Clearly I needed my notifications to work again.
First I tried the Provider for Google Calendar extension, but every time I attempted to add the calendar I got a "This version of the provider has expired, please update to the latest version." message. Googling around a bit, I found that this is not uncommon for people trying to use the extension with corporate calendars. Since our calendar is authenticated in a Red Hat-specific manner, I suspect it's just not compatible with the extension. Bummer.
I looked into other options. I tried importing the ical version (I just want notifications of events, so if it's read-only that's okay with me), but I couldn't find a way to make that work without setting my entire calendar public, which seemed less than secure. I considered trying a different browser for just the calendar window, but given how resource hungry browsers have become I didn't love that idea, and I'm not sure whether it would have fixed the problem anyway.
Finally I ran across a mention of Google Calendar Tab. This just opens Google Calendar in a Thunderbird tab, and it seemed like the notifications were done via Thunderbird so they were more likely to work. Lo and behold, they do!
But this wasn't without its issues either. By default this extension still just points at www.google.com/calendar, which thanks to the mobification of the internet now points at a mobile-friendly page that kind of sucks for desktop use. Fortunately, there's a solution to that too. In the extension preferences (Tools->Add Ons then Extensions->Google Calendar Tab->Preferences) you can set a custom url. Setting it to https://www.google.com/calendar/render
gives you the full fat desktop version of the site.
So it's not perfect, but I think it will work for me and I figured I'd write up my experience in case it helps someone else in a similar situation.