At first, I couldn't quite put my finger on it but, after using the browser for a few minutes, I finally realized what was not quite right about what I was seeing; IT WAS HUGE! After checking all the settings I could think of (zoom level, default font size) and finding everything as I expected it, I decided to fire up Chrome to verify I wasn't just going insane.
So, off to my favorite troubleshooting tool I went (http://www.google.com) and I searched for "Firefox update resolution change" which led me to a WONDERFUL thread on the mozillaZine forums (new firefox update caused weird browser resolution) that contained a magic post by member rais with the following information (paraphrased and annotated by me):
(if running Firefox 22)
- Enter about:config in the Awesome Bar (address bar) and press Enter. If you receive a warning about voiding your warranty, it's OK, just click through it.
- In the Search box, type rpx to find the layout.css.devPixelsPerPx setting.
- Right-click the setting and select Modify (or double-click the setting) to reveal the Enter string value dialog box.
- Enter 1.0 in the text box and press OK, you'll see the changes instantly
And to Mozilla, PLEASE FIX WHATEVER IT IS THAT CAUSED THIS IN YOUR NEXT UPDATE!!
UPDATE: After some further research, I've found out that this change was implemented to be more in line with Windows font sizing. In fact, I was forced to look into it further when I opened up IE (on Windows 8) and noticed that its content was also quite large.
So... The easy answer is turn your font size down (it's defaulted to 125% on Windows 8) if you don't like the look of your browsers.
P.S. Chrome has not followed suite with font size / DPI settings as of yet so it is unaffected by all of this as far as I know.