I'm a little confused because Mozilla isn't an email provider. It's a community that produces free (open source) software. One of its programs is Thunderbird, which is an email client (which enables you to access mail provided by a totally separate email provider, such as Gmail or your ISP).
So I'm only guessing that you're actually using Thunderbird. The instructions for altering the settings for displayed emails are here
http://kb.mozillazine.org/Font_settings_in_Thunderbird
but one of the things that Mozilla isn't very good at is explaining things to a casual (non-expert) user of its software. I'll try to simplify things:
Go to Tools -> Options -> Display -> Formatting -> Fonts
The important thing to look for is 'Minimum font size'. Adjust it to something sensible, such as 12 or 14 point (although the best setting for you will depend upon the size & resolution of your monitor - and how good your eyes are!). Then, even if someone sends you a message using a really tiny font, Thunderbird will automatically make it bigger for you.
You can play with the other options as well of course. You can't damage anything by doing so; if you don't like the results simply switch back again.
Chris