It is an oath authentication page and is served by Google to Thunderbird users that they have decided need to re-authenticate with them manually. The most common cause of this just popping up is the use of a VPN masking your location, however getting a new version of Thunderbird, a new desktop machine or even just accessing google from a different network or location could trigger their "you might be being hacked" software and impose additional authentication to check. Things like the VPN says you are in Mexico and your phone is connecting at the same time without a VPN from Canada makes providers justifiably jumpy. Note also that when Thunderbird updates some third party antivirus/software firewalls basically loose the plot and block internet access, so that is also a possibility.