In case you all have not seen it, there are a couple of very good extensions for Firefox that give great flexibility with visual styles.
Better Gmail 2 is an extension that gives you the ability to use extra macros, force secure (https) logins, and change the layout of GMail (including GMail using Google Apps for your domain). The Redesigned Skin is especially good. It also includes HTML signatures for GMail, which allow you to set up an auto html signature for your GMail account - very nice!
So I am looking at the new skin, wondering how to increase the font size, and I see that there is another extension I could use called
Stylish. It allows you to have stylesheets over the top of webpages. I used the
GMail Redesigned style instead of checking that option on Better GMail, and then I also got the style
Bigger Fonts to make it bigger. Then I added
Google in Segui UI to get the font I wanted. I liked it so much, I added the PB domain to the style sheet, and now I get Segui UI as the font on the PB. All very nice!