Fonts when reading and composing

Question

When I receive a new message, the font is extremely small. I have to hit the "A+" button to get it large enough to read. I can deal with that but would rather fix the problem.
The second issue is when I compose a message. Again, the font is so tiny I can hardly read it. I have the compose new message text set at Arial 10.
I've sent myself a message and opened it up in Thunderbird and it looks fine.
How do you make the fonts in messages larger without having to hit the "A+" button?

Answer 1

Thanks for taking the time to answer Claudio.
I understand what you are saying but these documents are ones I've been accessing every week for the last year and have only just started getting the problem.
I also notice that the ones that I don't have a problem with were created with a different program, earlier than the others as the guy who produces them had changed the program he uses.
If I haven't already said, I have successfully opened one of the problem ones and it was OK, closed it and re-opened it and I get the Abobe font instead of the Times New Roman.