Firefox is broken. v3.5 does this. The GIF image is correct, but firefox displayes it wrong.
However you don't seem to want the virtual canvas, so using +repage to remove the virtual canvas will fix your specific problem.
I reported the GIF issue to the firefox bugs list...
GIF with larger "Logical screen: Width and Height displays incorrectly
https://bugzilla.mozilla.org/show_bug.cgi?id=487562
I even had others confirm the bug, but firefox developers have been unresponsive.
In an animation only the first image goes wrong. If the first image fully defines the canvas then there is no problems.