Thank you, Bushfiva. So I may be a bit premature! I have been pleased with PaleMoon, as I've said, and my technical issues with Thunderbird (as with Firefox) are not the whole story so I am disposed towards trying FossaMail provided its technical status is no worse than Thunderbird. I can see that I need to find out more - particularly if I can benefit from the personal experience of those who know more.
I'm not likely to benefit much from the inclusion of Lightning because I use the ReminderFox extension with great happiness and I'm a 32 bit pilot at the moment, anyway.