That is the Tasks list accessed via the 'Menu Bar' toolbar - 'View' > 'Tasks' Customise the Unified Toolbar and put 'Tasks' onto the 'Calendar' tab, so then you can quickly switch to 'Tasks' window. It is identical to the same as used in version 102* When you single click to highlight a Task it tells you the info about the selected task in the lower section. If you want to update eg: Category then you can use the options to perform a quick update without needing to open the entire Task window saving time and reducing the number of clicks required. If you want to open in a new window or maybe you have set it up to open in a new tab, then double click on the Task in the list.

However, I'm getting a problem when it comes to reducing the height of the lower pane. The double headed grab icon is not vertical, it's horizontal. In 102*, I can grab the top edge and lower it to the point of removal from view. Maybe there is a bug reported, I'll check it out, but if not then I'll get it reported.

Unfortunately I was unable to customize the toolbar. I'll wait for the next update. I'm facing the same problem as you to reduce the height of the bottom panel. Thanks for answering me!

I created a bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1862322

Ah that's great. Good idea. Thank you very much!