yes it is a pain. and you have correctly identified the issue. You move the cursor "out of bounds" and it is no longer in an area that contains any formatting information at all.

My personal workaround as I like to fiddle in my mail a lot when writing them is to press enter about 6 or 7 times as soon as I get to the write window and then use the mouse to move the pointer back to the first line. It is not foolproof, but is stops most of the annoyances.

This add-on https://freeshell.de/~kaosmos/quoteandcomposemanager-en.html has a stabilise font option that is also quite helpful as it wraps the entire massage is a font tag so it is much harder to go "out of bounds"

Need install instructions http://chrisramsden.vfast.co.uk/3_How_to_install_Add-ons_in_Thunderbird.html

Thank You! The first suggestion appears to be working. I got your response on email I am 'abandoning' so I did not pick up sooner. It came in on rlihly@earthlink.net.

I will try the Addon later, but for now, this has stopped the annoyance with losing the format.

Gratefully rita@ritaihly.com

That is the email you have registered on this forum... if your abandoning it you might want to update your profile here so at some future date emails don't get lost if you ask another question after the account is finally abandoned.