While working on a PHP script I encountered a problem which was working fine on IE but not on Firefox.
I tried to access these elements by getElementById but that did not work either thereby causing me to think that it was impossible that the browser could not access an element which was visible on the screen. I Googled and found that this problem had been encountered by others as well.
Firefox checks for formatting and layout of tags very carefully and does not recognize elements if they do not follow convention. For e.g. the start tag must have an end tag. This was precisely the case with me. I was using <strong> to make the text stand out. Problem was that I had used an extra </strong> tag thus causing the problem on Firefox. I removed the extra tag and tried again. Voila it worked. However I had to use getElementById to access the elements. So, I made the changes and it started to work fine on both IE and Firefox and problem was resolved.