Got this email from Google after trying to log into Thunderbird. Is Thunderbird in-secure?
 

Sign-in attempt prevented

Hi Ajwad,
Someone just tried to sign in to your Google Account _________ from an app that doesn't meet modern security standards.
Details:
Thursday, February 18, 2016 6:15 PM )
We strongly recommend that you use a secure app, like Gmail, to access your account. All apps made by Google meet these security standards. Using a less secure app, on the other hand, could leave your account vulnerable. Learn more.

Google stopped this sign-in attempt, but you should review your recently used devices:

Using Mint 17.1 Rebecca

Looks like you need a very recent version of TB
https://bugzilla.mozilla.org/show_bug.cgi?id=849540

Hi...

This error might have to do with Google's OAuth requirement in which a fix was first introduced in (Thunderbird) version 38, from what I understand. Which version are you running? If it's older than version 38, first delete your email account information in Thunderbird, then upgrade Thunderbird using Synaptic (if a newer version is offered via the repositories, then re-enter your email account information in the new version. :)

Regards...