It's built into TB now, not an add-on, but you can remove menus etc. by right-clicking the calendar in the list in the calendar tab, Properties, uncheck 'Enable...'.

Thanks @sfhowes. I was able to remove the "Test" calendar but I can't get rid of the menu items as you suggested. I still have "Events & Tasks" in the menus and the icons for calendar & todos/reminders.

I'm on a Mac so I don't know if that changes the suggestion?

The Events & Tasks in the Menu Bar disappear after restarting TB (see pictures), but to hide the calendar and task icons, use the css code given here.

Hmm. I don't have that gear/option available by right/ctrl clicking. :-( I can't find what you have in your screenshot.

I have problems with scrolling in message pane too.

TB78 isn't ready for prime time yet.

On a mac, Options is Preferences. My picture just shows Options in a separate tab.

Thanks for your help sfhowes. I appreciate that people are volunteering to help sort out user issues. It can't be an easy task.

I thought I'd give an update to close off this issue.

I gave up on v78 today. The message pane is screwed up not showing the status line and the address lookup n new messages is not working. This update is abysmal.

I downgraded to version 68 using [./Thunderbird.app/Contents/MacOS/thunderbird --allow-downgrade] because there were changes to the Profile that wouldn't allow v68 to start normally. If I have to, I'll use the command line to start TB until I have time to figure out how to get my profile available again.

So for the time being I'll stick to the older version. Hopefully v78 will get fixed soon. I think TB needs more beta testers.