Probably configuring with a read only file (be it local or on a web site). Most web based ICS file can be considered read only and therefore you get updated when it changes remotely but you can't change it in Thunderbird.

So what provider are you getting the data from?

I’m not sure what you mean by “provider”. I’m really new at this just pretty much lucked into getting this far. How can I make iCal or thunderbird both read and write. I’d love to be able to schedule appointments in thunderbird and have it show up in ical.

I went away and googled ical. So your provider appears to be apple, although the ical abbreviation has many interpretations, just only one related to calendars.

So I assume you are asking how to connect an apple icloud calendar to Thunderbird. My google search returned this URL https://ict4g.net/adolfo/notes/admin/determining-url-of-caldav-calendars.html

Did you use one of the links in the document to get your calendar in Thunderbird. The one with the ICS is my guess. But please check it carefully

If you did not set up the calendar using a URL, how did you do it?