According to this page
https://support.google.com/accounts/answer/6010255
To help keep Google Accounts through work, school, or other groups more secure, we block some less secure apps from using them. If you have this kind of account, you’ll see a "Password incorrect" error when trying to sign in.
Are you accessing through work, school, or another group?
I found the less secure apps option pictured below. Mine is turned off and Thunderbird still works correctly.
When I setup Thunderbird to use gmail, I was pesented with a option that allowed Thunderbird to have access to my gmail account.
Now at this location
https://myaccount.google.com/permissions
I see this
If you don't see the same in your gmail account settings, you might try removing your gmail account from Thunderbird, then add it again to see if you get a option to add Thunderbird as trusted third party app.