Hi Bruce Wheeler, I'm not familiar with Waterfox. It isn't a Mozilla product. Here's the subReddit which they use for their Support Forum: https://www.reddit.com/r/waterfox/

Thanks for your suggestion. When I click on Waterfox Help it takes me here:

https://support.mozilla.org/en-US/products/firefox?as=u&utm_source=inproduct

Looks Mozilla to me... Anyway, FWIW, I reinstalled both Waterfox - newest V, and Thunderbird - no difference in my problem

OK Select Menu -> Options On the General page, under Applications Set mailto to Always Ask

When you use the Email Link a Launch Application dialog will open. Select the mail application that you wish to use.

Any difference?

I did it, no change. I click on the email the page button, the hourglass shows up, and that's it.

It could be a problem in your Profile. Try creating a new 'Test Profile' to see if the problem persists there.

Otherwise, you may need to do a Refresh.

Followup: it was a Thunderbird problem. Latest version fixed the problem. Thnx for your help.

Thank you for taking the time to let us know.

Browse Safe!