Is the 'Formatting Bar' visible ?

Click on 'Write' click on 'View' > 'Toolbars' select 'Formatting Bar'

No it is not showing on the particular message. Although it to shows on a new message, not the one I want to format. The work around is a cut and paste into a new message. Annoying but I can get it to work. I have the formatting bar ticked but it is not showing. Thanks for your help.

If you received a message that was composed using plain text, then Thunderbird will reply in kind, using plain text. In this case, the most pragmatic solution is often just to do as you have done, copy and paste into a new message. Unfortunately, this breaks any threading because the message you send is a new message and doesn't refer to the original.

I belive an upcoming version of Thunderbird will allow you to set it to use your own preference rather than retaining the mode used in the original message.

Until then, you could look at a couple of add-ons.

https://freeshell.de/~kaosmos/changequote-en.html https://freeshell.de/~kaosmos/quoteandcomposemanager-en.html

I think either of these will allow you to select html regardless of the styling of the original message.

Thank you. It is a relief to know I am not going crazy!

A thought. Try holding down "shift" when clicking "reply".

The shift key usually switches the composition mode to the non-default. So you need to decide whether or not the original message was in plain text or formatted using html, and use shift only when it was plain text.

If you get it wrong, no harm done. just cancel/abort and start over, this time doing the opposite thing with the shift key.