Hi Welcome to Sitepoint
Without full code its hard to give an accurate answer but Firefox is usually pretty consistent. (IE6 is usually 1px out with absolute positioning when using right and bottom and the distance covered is an odd number of pixels.
The first things to check would be:
Have you zeroed padding and margins on related elements?
Have you set a stacking context with position:relative on the parent?
Have you set a consistent line-height.
If its text that you are trying to position then text is always out between browsers just because they all handle it differently.