Thunderbird Could not connect to server

My wife's MacBook Pro running macOS 10.14.3 suddenly developed a problem with Thunderbird 60.7.0.

It cannot connect to any email server and produces an error "Could not connect to server" "The connection was ref..."

I have checked the email accounts, and can logon from Google Chrome on my iMac and on her computer.

What steps can we take to resolve this without losing existing emails?

Time Machine backup (about a week ago) and a fresh backup just now.
Time & Date OK.
I deleted Thunderbird app, and re-installed from a fresh download. Problem remains (as expected).

Solution 1:

Apologies for the question.

After further analysis I found ApolloSearch running and busily downloading. Killing this with MalwareBytes and updating to latest macOS seemed to "fix" the problem.

Not sure why this just seemed to affect Thunderbird, or how it got there.

It would still be handy to know how to kill Thunderbird without losing emails.