Firefox changes font sizes when you command-scroll. It’s sooooo annoying and I turned it off once before, but had to do it again after getting my ‘Book back from Apple.
Here’s the solution I finally dug up (again). I’m putting it here so I don’t forget it again, and hopefully make it easier for someone else to find.
To turn if off, go to about:config, and change
mousewheel.withmetakey.action to 0
mousewheel.withmetakey.sysnumlines to true.
Source: comment at http://www.macosxhints.com/article.php?story=20051201120732892