There kinda is set of workarounds for this:

Setup your Thunderbird instance to move to trash NOT immediately delete, then you can restore them singularly or en masse from there.

Setup local folders where all things you delete go on your local or networked drive ( i.e.nas, company/college share drive) and similarly restore from there.

or just use ctrl+Z - the one-handed universal undo which takes no screen space

There is always Edit/Undo Delete Message (Ctrl+Z), and you can create a button for Undo last change/action.

Thanks very much all who pointed out the ^z solution. It works! I strongly suspect that a majority of users who may be used to using ^z all the time while eg editing a document have no idea it works for things like deleting an email, moving email to another folder and doubtless many other things too. I suggest Mozilla makes more of this power feature!