Crashing desktop and Thunderbird

In the past week or so, I have been seeing a lot more occasions of desktop crash....the desktop mountain background scene disappears and presents me with the login page. That usually fixes it (logging back in); but it has been annoying.
Now this morning, my Thunderbird client has refused to open at all and just crashes the whole desktop back to the login page. I realise this may be something to do with Thunderbird and not Zorin, but until I've posed the question here I can't take the next step.
I also seem to have difficulty with software updates; the updater regularly says it can't connect to the repository, and yet I have a positive internet connection. I am wondering whether I am missing some important updates.
I have Zorin 16 Pro installed.
Any thoughts gratefully received.

First try to get all the updates by:

sudo apt update && sudo apt upgrade

Many thanks for the code, I'll run that today.

Have done this; with one unexpected result, to do with my Brave browser, but it seems to suggest that everything is up to date:

The Brave error you can ignore. Try start thunderbird via the terminal and see if any useful comes out.

Well whaddyaknow, $ thunderbird command opened the application up no problem. Had never really thought to do it via the command line; but now the question is why the shortcut icon on the desktop doesn't work.
I'll do it a few more times to see that it still works ok; if so, the desktop icon will probably be toast, although the problem then remains unsolved.

You can add/edit the shortcut icon with Zorins menu editor.

Thanks for that, will have a look at the menu editor....never looked at that before. Main thing is at least I can open the application now, so will tick that off as solved.