re :With the Supernove release you moved the search bar and action icon into the top of the window.

That's the 'Unified Toolbar'

BTW - If you want the Menu Bar toolbar on top then use info at this link: https://support.mozilla.org/en-US/questions/1422908

re :I configured to create new appointments in a new tab, not an extra window. But in this appointment tab the action buttons are in a seperate action bar below the tabs for email and calendar and not next to the search bar

That's because the 'Unified Toolbar' can be customised for: 'Mail' , 'Address Book', 'Calendar', 'Tasks', 'Chat' and 'Settings'. But not an Event. Hence you only see the specific toolbar for Event in the Event tab. This is necessary because that view in the 'Event' Tab must also be available if viewed in a separate window which does not contain a 'Unified Toolbar'.

Why a new design if it is not consistently implemented? I don't need 27 different toolbars depending on which module is used in which mode. If I use something in tab mode, then the associated buttons should be in the 'Unified Toolbar', if I use it in 'separate window' mode, the buttons must be in the window. Otherwise you can leave it alone.

yeah, supernova is at best a starting point. Personally I think those responsible bit of much more than they could chew with supernova and by the time they worked out just what they had done, it was too big a job to back out of it.

Each release will see improvement in the consistency of the user interface, at least I hope so. But really they still have not done implementing the card view to use instead of the list view for emails. See https://developer.thunderbird.net/planning/roadmap

Note especially the bit about "out of scope" which includes the entire calendar.

I quote part of the link here

As we focus on these primary objectives with the goal of bringing them to full completion, as well as making sure we have enough resources to tackle bug reports and regressions, we won't be dedicating any major commitment to the following areas of the application during this year. Calendar Chat Tasks Settings Compose Search and everything else...

If you have feedback or ideas, there is a link on Thunderbird's help menu for that. It will take you to this location. https://connect.mozilla.org

Why a new design if it is not consistently implemented? I don't need 27 different toolbars depending on which module is used in which mode. If I use something in tab mode, then the associated buttons should be in the 'Unified Toolbar', if I use it in 'separate window' mode, the buttons must be in the window. Otherwise you can leave it alone.

I'm not disagreeing with you. I'm just stating how things are at the moment. I'm just another user like yourself. The developers do not get involved with the Support Forum, so there's no point in stating what you want to happen in this forum. If I knew some code which would fix it via using a 'userChrome.css' file then I would have offered the info, but in this case I don't believe it's possible.

Hopefully, as Matt has stated, the developers will get around to improving things as time goes by.