I have a form in a fancy box. In FF3, the form is a little bigger than Safari because the text inputs have a larger height. I tried explicitly setting height with px and em but nothing seems to work. Any idea how to fix this?
You can't fix this, at least not completely. Form fields will always display slightly different depending on the browser, operating system and system settings.
Different browsers will have different default values for font family, font size, padding and margin for the fields. You can specify values for all these, that will get you closer.
You can place each form field inside another element (for example a div) that you give a specific height that is some pixels larger than the field, that way there is some room for differences while you get a total size that is the same across browsers.
Check that you haven't set a minimum font-size in Firefox - that has caught me out on a few occasions.
It's in the advanced section of the font options.