Released on October 31, Thunderbird 60.3 fixes a handful of bugs — some of which are security-related — affecting multiple versions and platforms.

From the security advisory: In general, these flaws cannot be exploited through email in the Thunderbird product because scripting is disabled when reading mail, but are potentially risks in browser or browser-like contexts. What they seem to be saying is that these vulnerabilities cannot be exploited through the act of opening and reading email in Thunderbird. As for the part about browser-like contexts, well, that’s not at all clear. What contexts?

You can update your install of Thunderbird by clicking its hamburger menu button at the top right. Click the small arrow to the right of Help, then click About Mozilla Thunderbird. The About dialog should show your current version and offer an update if one is available.