How do I check which version of Thunderbird I have?

At the top right of the Thunderbird window, click the menu button , then select Help, and then About Thunderbird.

The latest released Thunderbird version is 38.3.0. The version provided by your distribution my differ.

and is it secure?

Assuming you do have the latest version it is as secure as it can be. There are security updates and bug fixes every 6 weeks.

What makes you believe it isn't secure?

davidgarside1 said

I have read that the latter is not secure.

Are you referring to the article mentioning Thunderbird in Computer Active magazine #458 ?

The author misunderstands the purpose of https://www.mozilla.org/security/known-vulnerabilities/thunderbird/ and claims Thunderbird is full of holes simply because the Thunderbird council devs are much more open compared to most others who keep security vulnerabilities more hidden unless there is critical vulnerability case to scare people to update to current version say.

Makes one wonder if the author or magazine was paid by Microsoft to write that misleading misinformation part about Thunderbird so people would move from Thunderbird to Outlook mentioned in article.

Yes that was where I read it. Thanks.

to balance the ledger, perhaps have a look that this award Thunderbird received the same month. http://www.infoworld.com/article/2982630/open-source-tools/bossie-awards-2015-the-best-open-source-desktop-and-mobile-software.html#slide4

Thunderbird has many issues, but lack of a dedicated volunteer development team is not one of them. There are not enough of them, but security is not being compromised.