|Thank you Ed,|
I did run the Superantispyware which found 5 adware tracking cookies, and one other thing called Rogue.Agent/Gen-Nullo[dll] (C:Windows/System32/ODBCCR32.dll).
They all got quaranteened, I will delete them to make sure they do not come back.
I then opened my Mozilla Thunderbird. The messages which already had ivisible text, are still invisible, but when I reply or start a new message, now the text is visible.
So, thank you for your help.