Do you get any error message? Did you kill thunderbird.exe in task manager prior to attempting to start it?

There's no error message. Just doesn't open. It will only open two ways: 1.) If I haven't tried to open it since last reboot it will open in safe mode. Or... 2.) If I've tried and failed to open, I must end Thunderbird process in Task Manager/Processes, then it will open in safe mode.

FWIW, I've noticed that before I end the thunderbird.exe process, it continues growing in size by a few kb per second - indefinitely, as near as I can tell. Thanks for your interest.

Can you post your Troubleshooting Information? Help (Alt-H) - Troubleshooting Information

Application Basics

Safe mode disables any extensions you have installed. Your only extension is Lightning, the Thunderbird calendar add-on. Did you use Lightning before? It ships with Thunderbird as of Thunderbird 38, but if you don't need it, you can safely uninstall it.

Then I probably need to uninstall it. Prior to this the only serious problem I've ever had with Thunderbird involved Lightning. I used and enjoyed that at one time, but it hanged on me and.... Come to think of it, what's happening now was the result then.

I wish I could use Lightning, but apparently it's the culprit. Anyway, these days I'm making do with Windows Calendar, so, how do I uninstall Lightning?

Thanks in advance.

Lightning should work just fine, but something may be broken in your profile. Uninstalling Lightning would be a temporary workaround if your're willing to continue troubleshooting the problem. Up to you.

You can uninstall Lightning via the Add-on Manger. Tools (Alt-T) - Add-ons - Extensions Press 'Remove'.

If I could make Lightning work, I'd be thrilled. What should I do?

The first thing I'd try is to uninstall Lightning via Add-on Manager. Then download it from AMO and follow the instructions to install it again. https://addons.mozilla.org/en-US/thunderbird/addon/lightning/

What calendar(s) do you have - local or network?

Been trying to find how to uninstall via add-ons mgr. ....

https://support.mozilla.org/en-US/questions/1072909#answer-755604

Tools|Add-ons

Select Lightning

Click "Remove".

I missed the "extensions" step. Got it. Lightning removed - Thunderbird opens normally.

Okay, that was a fruitful experiment in that I now know my Thunderbird has to be opened in safe mode if Lightning is installed.

I re-uninstalled Lightning and now Thunderbird opens normally again. Apparently a relationship between Lightning and me isn't meant to be.

Thanks for the help.

As previously suggested you can try to install it again and see if it's getting any better then.

I probably didn't make clear enough that I did the reinstall. That was the experiment. All it did was disable Thunderbird anew, which led me to speculate that perhaps a relationship between Lightning and me isn't meant to be.