Cant access mail. Is it Thunderbird or Google problem?

Have been using Linux for some years now since Ubuntu 10.04, and never had any problem receiving mail. I've recently bought an HP 64 bit computer and upgraded to Ubuntu 18.04 Since then Thunderbird appears to be working, but when I click on Inbox, I get a Google security window popping up. This asks for email address and password, next window requests mobile phone number to send a security code to. I do as required and get a message confirming success. When I click inbox again, the same scenario recurs, (about 15 times now). I can still access email from my phone. Sincerely hope I'm not wasting anyone's time here, as I suspect the problem lies with Google. I have contacted Google, but had no response after a week. Any advice or assistance would be greatly appreciated,

PS: Everything else appears to be working normally.