>Update 2: Setting the font explicitly to Calibri is the same as leaving it blank. Calibri 14 is smaller than Arial 13 and definitely not as clearly rendered in FireFox for me.

That is the default: Calibri, Arial, Helvetica, sans-serif;

So, if you want Calibri, there is no need to set it.

>Update 3: Arial 14 is a bit too big for the messages, and is ugly bolded in the top pane.

Arial is what you add before. I was actually: Arial, Helvetica, sans-serif;

So, setting it back to Arial with the proper font size will give you exactly what you had before.