Why I can't see this immages using img tag in FireFox?

I am not so confident with HTML and CSS and I have the following problem.

Into a table I have something like this:

<td style="border-bottom: 1px solid #76818a !important;  width: 10px; text-align: center; padding: 0 !important;">

    <div style="width: 90px;">

        <img alt="su" src="img\freccia_up.gif"
             onclick="Javascript: spostaSu();"> <br> 

        <img
            alt="giu" src="img\freccia_down.gif"
            onclick="Javascript: spostaGiu();"> <br> <br> <br>

        <img alt="nascondi" src="img\freccia_dx.gif"
             onclick="Javascript: eliminaColonna()"> <br> <img
            alt="visualizza" src="img\freccia_sn.gif"
            onclick="Javascript: aggiungiColonna()">

    </div>

</td>

As you can see it show some .gif immages stored into the img path of my project.

The problem is that using Explorer and Chrom it works fine (these immages are correctly showd into the page) but using FireFox the immages are not rendered and instead these immages it is showed the value of the related alt attribute of the tag.

Why I have this behavior? What can I do to try to solve and correctly show these immages also using FireFox?

Tnx