Any idea how or why you came to be using a beta version?

If you want to beta test then thank you. But if you do, you need to realise that you're using non-mainstream software and will have to do some extra work.

In your position I'd download and install TB43 (rather than following an upgrade route) since Lightning will come along with it.

If you want to go back to the main release channel, download Thunderbird from here:

https://www.mozilla.org/en-US/thunderbird/all/

…and reinstall.

Try the beta version of the addon listed at https://developer.mozilla.org/en-US/docs/Mozilla/Calendar/Calendar_Versions

I'm unsure if it will work, because you are using TB43 and the version listed is for TB44, which we haven't yet released.

Please tell us your results

Wayne, Thanks, I checked the link. Since TB43 Windows is reporting it is "Daily" I tried lightning-4.8a1.en-US.win32.xpi and lightning-4.8a1.en-US.win64.xpi. Neither worked, so I'll try Zenos solution next, but thank you for suggesting a solution to try.

Zenos, Last I recall, I installed TB beta to test memory usage from a developer to see if changes improved the memory situation on my 4GB laptop. TB and Firefox together were were saturating 4GB of RAM in less than one day of usage on my laptop. Since my laptop isn't just a TB/FF machine (I also run office tools, compilers, CAD) and Mozilla products now force users to upgrade to the latest "one size fits all" solution, this wasn't acceptable. Since a version of Lightning was available which worked under "Beta", I forgot about this issue. Now that no Lightning is available for this release, now is a good time to switch back to release. Since I either never knew, or have since forgotten how to do this, thank you for providing a link. If this works I'll return and mark this solved.

Thanks guys!

Craig

Following Zenos link I ended up with Thunderbird 38.5.0. Then I had to reinstall Lightning 4.0.4.1.

I'm back up and running with Lightning!

again, Thanks!

The Daily refers to the most unstable development channel which currently has Thunderbird 46.0a1 on it like how Firefox 46.0a1 is on Nightly channel.

Thunderbird: Release < Beta (44.0b1) < Earlybird (45.0a2) (Aurora) < Daily (46.0a1).

Firefox: Release (43.0.3 current) < Beta (44.0b4 currently), Aurora 45.0a2 (also called Developers Edition) < Nightly 46.0.1

The current Beta build for Thunderbird is actually 44.0b1

If it says you have a Daily channel build then it may be a old random build from when Tb 43.0a1 was on Daily channel.

Thunderbird is doing Release every seven versions (extended support release fashion) so the next major Release will be Thunderbird 45.0

Glad you are up and running

James, FYI - 44.0b1 has not even been built yet. We are not always able to meet the strict calendar schedule that Firefox is able to churn out. Indeed, 43.0b1 only recently became available.