Form page jumps using Firefox

I have a form using the plugin to calculate sums from a number of spinbox fields. They work acceptably now with Chrome and Safari, but with Firefox the form jumps to the top of the page each time you try to increase a value in a spin box farther down the page. (It doesn’t preserve the decimal places sums, either. That is, if you add 40.00 and 40.00 the result displays as 80.)