Either ignore the gmail hype and turn less secure apps back on or if using IMAP and SMTP change your authentication method from Normal Password to Oauth2 in account settings in Thunderbird. If using POP you have to turn on Less Secure Apps or use the gmail webmail interface like they are trying to force you to do in the first place.

how do I turn on less secure apps?

How do I turn less secure apps?

You go to the gmail site and log into your account there. It is an option at gmail.

What is the gmail webmail interface and why don't you recommend it?

Hi lstallard, Airmail's post was marked as the solution by the person who asked the original question (LarryDean920). Your issue may have similar symptoms, but it is likely a different cause/solution. You should use https://support.mozilla.org/en-US/questions/new where volunteers can get more details about your setup.

Because this thread is solved and more people might start using it for their own questions, I'm going to lock it.