Why does the Admin Editor Font Sizes display different in Internet Explorer and Firefox?
When changing fonts the editor will display a suggested size: 1(8 pt), 2(10 pt), 3(12 pt), 4(14 pt), 5(18 pt).
However the editor actually uses Absolute Font Sizes: xx-small, x-small, small, medium, and large.
So the editor displays the #'s and pt's but uses an Absolute Font Size in the HTML upon creation. eg.
xx-small = 1(8 pt)
x-small = 2(10 pt)
small = 3(12 pt)
medium = 4(14 pt)
large = 5(18 pt)

CSS specification sets a scaling factor that each next size up should increase by about 1.2pt. The downside is that it does not specify what size it should start at. so Internet Explorer will start off with xx-small = (1 8pt) as the editor suggests but FireFox starts off with xx-small set to (10pt). So FF will always look one size higher when displayed. Browser vendors could have picked the same values for these keywords but they did not.

Why does the editor automatically change my font tags?
The <font> tag has been deprecated. This means that the tag is no longer a part of the HTML specification. While browsers will still support it, most likely for a long time, it's a good idea to switch to the alternative.  Cascading Style Sheets. CSS can do all the same things that the <font> tag can do, plus a lot more which is why  Absolute Font Sizes within a <span> are used as mentioned above. With CSS, you can change the font size relative to the base font size in the browser.  When it comes right down to it, deprecating the font tag is good. With CSS to fill in the gaps and add extra tools to manipulate your text even more than the font tag ever did.