Change layout.css.devPixelsPerPx in Options/General/Indexing/Config. editor:

https://support.mozilla.org/en-US/questions/1269361

I did find where to change it, but I don't know what value to type in the box. It has -1.0 . I would like the size to be like 11 or 12 points. Can you please tell me what I need to change the value to get the desired size?

You can't set a specific point size unless you apply a userChrome file (described in the above link), but if the preference is increased gradually from 1.2, you can see the effect:

https://support.mozilla.org/en-US/questions/1269361#answer-1254162

Thank you so much!! Your instructions were perfect. I went with 1.5 and it's exactly what I wanted. I am very impressed with the level of help that is given in this forum. I've had two questions and both were answered promptly and with the solution to the problem.

I tried at least half a dozen other email programs in the past month. was looking for a replacement for Incredimail which I've used 1999. I am happy with just a simple email program where I can send and receive email without a ton of frills, etc. Have also used FireFox ever since it's inception.

Again, thank you for making it easy for me to make the necessary changes. At 83, we sometimes need a little help.