If you mean wrapping in the message compose window isn't right, select Preferences/Advanced/General/Config. editor and copy mailnews.wraplength into Search. The default value is 72, but you can right-click, Modify, and enter a different value, or 0 to wrap to the window width (plain text messages).

In Preferences/Composition/General, uncheck the box 'Use Paragraph format...' to disable double-spaces between paragraphs.

Like with a lot of these suggestions your 'paths' do not line up with what I see on my version of Thunderbird:

Such as  : Preferences/Advanced/General/Config

I get as far as 'advanced' and then mine is completely different with no options I can see for adjusting text wrapping  ??

As for: ''Preferences/Composition/General, uncheck the box 'Use Paragraph format...' to disable double-spaces:"

Who would have thought that ? What planet are these designers/coders from ?

And why did it suddenly change this week - an update maybe ? And why do updates for Thunderbird usually remove or change useful features - just my observation over the years..maybe too keen to rush out new features so we actually loose the useful ones.

See the attached picture for the location of Config. editor.

Thanks but that just gives me a list of thousands of options - most on default, the ones in bold as modified I haven't modified them and nothing like wrap text that I can see - or any idea what to do if I did find it in this list of thousands of options.

I've had years of this with Thunderbird - nothing much needs changing over the years with email but Thunderbird is always changing - each time like throwing a pack of cards in the air.

Thinking is time I went and bought myself something that isn't like throwing a pack of cards in the air every time it updates.