Click on the menubar (three horizontal lines) and select preferences (or you can just do command-,). Then, in the searchbar at the top, type in "fonts". From here you are able to change the font, make the size bigger and change the colour. For different languages and more specific fonts, click on "Advanced". I hope this solves your problem.

You could also check the solution in this thread: https://support.mozilla.org/questions/1239467

Thank you both for your responses. This solution of jscher2000 helps me a lot. The standard value was -1.0. I put the value on 1.25. The same trick works also in Thunderbird! Thank you very much mr. jscher2000!