hello ob9s, i have no multitouch-pad myself so i can't test if this will actually work, but it's probably worth a try: enter about:config into the firefox location bar (confirm the info message in case it shows up) and search for the preferences named browser.gesture.pinch.in & browser.gesture.pinch.out - doubleclick those two entries and change their value to false. or alternatively you could try setting zoom.maxPercent & zoom.minPercent to 100 ...

got it fixed, thanks for the help! your second suggestion seems to have done the trick.