Zooming with the CTRL+Mouse Wheel Up/Down has been in inversed in Firefox 3. In version 2 you zoomed in (enlarged the text) with CTRL+Mouse Wheel Down and zoomed out with CTRL+Mouse Wheel Up, its now in Firefox 3 the other way round.
The revert back to the old behaviour, change
mousewheel.withcontrolkey.numlines = 1
mousewheel.withcontrolkey.numlines = -1
in about config.
To me “pulling” the page towards myself always felt much more natural to me than the other way round, but that’s just me.