What are your server settings?

https://support.mozilla.org/en-US/questions/1336913

Are you trying to connect through a non-WOW network?

Hi vansyckel, I have WOW email and when I called them this afternoon they said they had been getting calls today about email problems and that there was an outage and they were still working on. Perhaps the reason you and your wife could not connect? If I recall correctly there was a similar issue one day within the last several months when there was some type of "outage" that stopped the reception of email for a day or so, or maybe part of a day. Will see if the "outage" has been resolved by tomorrow. Regarding your comment about WOW support my experience has been that their front line support staff members don't have very deep technical knowledge. The WOW rep read off of a statement about problems today with 3rd party apps. I told her I also had a problem when sending from WOW web mail which is not a 3rd party client. But her knowledge of the situation only went as far as to say there was an outage, and that they were relaying reports like mine to the engineers to let them know that there were still problems.

seekinganswer, thanks for your comments. WOW also told me they were getting numerous call but all were from Thunderbird users so it was a Thunderbird issue. This made no sense to me because we had not changed anything and I'm sure many others had not changed anything Wednesday night and yet all our emails broke at the same time with WOW being the common thread. It is now Friday morning (36 hours later) and still broken. Our WOW web email seems to be working but we're missing all the functionality of Thunderbird to manage our email. Among there features is the use of identities which we always uses because I own the domain and all our email is addressed to domain based addresses and forwarded to the WOW accounts.

sfhowes, Neither my wife nor I have changed any email settings in months. We are on our home network attached to WOW. By their own admission, many Thunderbird users called in with the same connection issue. I know we didn't change any settings Wednesday night and I would guess that most of the other Thunderbird users that broke didn't change any settings either so the common thread is WOW email servers. I'm thinking WOW tweaked some setting on their servers that made our settings not work but they won't admit it. I've experimented changing a few of the settings in Thunderbird with no luck. I have to look for a new ISP.

I have to look for a new ISP.

ISP or mail provider. Seriously I have not used my ISP provided email account for years, simply because their service was so sub standard I moved elsewhere. They have, I understand, improved their service in recent years, but I have moved on. I use google and GMX, but there are also lots of paid providers who make it their job to provide high availability mail.