I want to delete 'offered' email addresses from T'bird's lists. This is easy in 'Collected Addresses' but there are other, often duplicated, lists such as 'Family' that include out of date info. If I open up the category I can access the addresses BUT when I highlight a specific address and right-click on it the drop-down menu has 'Delete' greyed out and therefore I can't delete it. An example is my wife's email addresses from her last two places of work - and she's now retired!
Looks like you have some external contact database connected to Thunderbird like Google contacts or Exchange. In that case it depends on the used plugin if deletion of contacts from such list is implemented. Try to go to the origin of the contact database and remove members from there.