The problem wasn't my firewall - since I was getting emails in and out.
The problem wasn't my passwords - it was failing before the password exchange.
The problem was... cacheing. My ISP only allows so many connections to it's mail server and I found that the Thunderbird was cacheing connections - which apparently means keeping the connections open.
To correct go to account setttings. Under each account select: Server Settings>Advanced...
Then change "Maximum number of server connections to cache" to 1. (Mine was set to 5)
I have no idea if the setting got changed on a thunderbird update or whether the functionality changed to start causing a problem.
Anyway setting the number to cache to 1 has stopped the problem.