Thanks for the attempt to port to the new firefox but this is nothing like it was prior. This simply opens a tab with your calendar in it which technically no add on is needed to do this.

I noticed you mentioned the new add ons don't allow customization like you had prior

May I suggest looking into the following

https://addons.mozilla.org/en-US/firefox/addon/checker-plus-for-calendar/?src=userprofile

It is what I have been using while waiting for your update. It lacks many of the features you offered in your previous version but it shows that add ons are still possible similar to the older version you had with simple pop ups etc