@slapzstick. It appears Firefox (at least on Mac) is adding an extra 1px padding to the text area in addition to whatever you specify; this means with certain inputs (such as your example text), the text in the text area will wrap a word before the pre does, so it gets out of sync. As a workaround, you can add a Firefox-only padding property to the text area (this will be ignored by other browsers:
padding: -moz-calc( 5px ) -moz-calc( 4px );
That fixes it. Strange, and annoying, bug in Firefox though.