what in the network link are you using to access that calendar?

The "url" was: googleapi://my.user.name@gmail.com/?calendar=1on2hba82bs0mll22709kmgs52q3jl8m%40import.calendar.google.com

I was unable to export that account from Thunderbird to my computer or do anything useful with it. Frustrating!

All of that link is for a google calendar, your question is about a yahoo calendar. So could you please clarify.

Matt-- I got that link by going to Thunderbird/calendar, and right clicking on the PROPERTIES of that calendar. The link appears under LOCATION (the other property is NAME).

   This may not really be a TB problem-- the Yahoo calendar seems to appear correctly in the TB/Lighning calendar.  However, I cannot export it from TB, something I tried to do in order to get it into my calendar app on my Android phone.  When I try to do that, it gags on that link, which is why I brought it up here.
  It is amazing (really embarrassing) to discover that after >20 years of Yahoo and Google calendars, getting them synced is still a problem.

The link is for Google, so your yahoo calendar can not appear using it. unless you are already importing your yahoo calendar to google. How about you just add a yahoo calendar using the yahoo link and see how it goes.

I never said or thought this was the correct link, but that is all TB provided under LOCATION when I right clicked the Yahoo calendar that is already attached to my TB.

   I guess I'll just have to give up on the idea that TB can be used by mere mortals to process mail from several services (gmail, yahoo, etc.)

I was dealing with someone that said they had something that did not work. Now I learn they want to actually add the calendar to Thunderbird, not fix the one they have. Just a lot different.

yahoo offer instructions here

Which tell you to use the link https://caldav.calendar.yahoo.com

So you add a calendar to Thunderbird by clicking on the new entry on the menu and selecting the calendar entry from the menu. Then you obviously choose to use the On the network entry as the data is not stored locally. Then you enter the link Yahoo provide in their help and complete the wizard. Be sure you actually know the user name and password on the yahoo servers when you are asked.