Just a quick note for anyone who may be working on something in Firefox and have the need to undo an action (cmd-z) then quickly hits redo (cmd-shift-z) and gets unexpected text zooming.

I initially thought it was some unknown accessibility feature in OS X but Zoom in the Universal Access control panel was already disabled. Then I checked Firefox's built in Zoom command (accessible through the View menu) but it uses a different shortcut key (cmd-+ and cmd--).

After much head scratching I was able to track it down to the Firefox Web Developer plug in. For some reason it's default shortcut key for zooming in is cmd-shift-z. I cleared both short cuts (zoom in and zoom out) in the Options dialog for the Web Developer Plug in and no more crazy zooming.