I answered a question from a new Firefox user about font sizes. She had just installed Firefox and noticed that the fonts were smaller than those that she was used to seeing in Internet Explorer.
The "problem" is that we, as users, can change our default fonts and font sizes in our web browsers. Firefox even offers more flexibility than does Internet Explorer.
In Internet Explorer, we can make a temporary change by clicking on View, Text Size, and then pick any of five different relative sizes. By relative, I mean that they have values like “larger” and “smaller,” which get applied to the current font size on the web page. Firefox has a similar function on its View menu.
To make changes to the default fonts in Internet Explorer, you go to Tools, Internet Options and select the General tab. Then, click on the Fonts button near the bottom of the dialog box. This will open a window where you can select the default font to be used for “web pages” and for other text boxes. Some of these give you font size choices, although most do not.
Firefox has more flexibility in your font size choices. In Firefox, go to Tools, Options. Then, on the dialog box, click on the Content tab and change the font size to the one you want (I use 16). You can click the Advanced button there to set some additional font defaults for other font types (sans-serif, monotype, etc).
Another good tip: In most programs, not just in web browsers, you can also hold down Control and roll the scrol-lwheel of your mouse to change the font size on the page you are currently displaying.
Firefox carries this changed font size (changed via the scroll-wheel) to subsequent web pages that you view during that session, but Internet Explorer only applies the change to that individual page.