Thunderbird Send button vanished after last Tbird update

Hi, Tbird 78.5.1 32 bits.
After the last update to this, the square Send button is no longer present. There used to be one in the blank space here when composing messages:


I've looked at fixes for missing Send buttons but they all relate to the toolbars - and there is a Send button there. I've tried resetting toolbars (Shift click Tbird icon...) but am starting to wonder if this is just a design change. There's no Send button in Customise, but I suspect that's not relevant to this particular Send button.

Any thoughts? Thanks.

If you mean to the left of From/To/Subject, I don't ever remember one being there?

Uninstall, download an earlier version and check
Thunderbird old versions - Windows

Thanks, it's the Send button that used to be in the blank space I'm missing. I am aware of and can use the Send button in the toolbar.

I'll look at the version on my other PC and post a screenshot when I get round to it.