Selenium / Firefox: Command ".click()" doesn't work with a found element

I tried to find a solution to this thing and I spent a lot of time, but it is almost imposible to me to do that.

The matter: I am using Selenium with Java in Firefox. I need to find an element (a listbox) and click on it. So, the code finds the element, but click action does not work. It works fine in Google Chrome every time, and just sometimes in Firefox (with the same Java code sometimes works, and sometimes does not).

There is the part of code with the element when the program enters on the page:

    <div id="size-btn" class="size-btn">
      <span class="selected-size">SELECCIONA TALLA </span>
      <div class="size-select" style="display: none;">
        <table>
          <tbody>
            <tr id="selecsize_2" class="product-size" data-ga-props="{action:'Seleccionar_Producto', opt_label:'Escoger_Talla'}" data-catentryid="1051607">
            <tr id="selecsize_3" class="product-size" data-ga-props="{action:'Seleccionar_Producto', opt_label:'Escoger_Talla'}" data-catentryid="1051608">
            <tr id="selecsize_4" class="product-size" data-ga-props="{action:'Seleccionar_Producto', opt_label:'Escoger_Talla'}" data-catentryid="1051609">
            <tr id="selecsize_5" class="product-size" data-ga-props="{action:'Seleccionar_Producto', opt_label:'Escoger_Talla'}" data-catentryid="1051610">
          </tbody>
        </table>
      <button class="size-guide gaViewEvent gaTrack" data-ga-props="{action:'Seleccionar_Talla', opt_label:'Guia_de_tallas'}" data-href="http://www.anyweb.com/webapp/wcs/stores/servlet/ProductGuideSizeAjaxView?catalogId=24052&categoryId=358056&langId=-5&productId=1047599&storeId=10701">Guía de tallas</button>
      </div>
    </div>

And there is the part of code that changes when the element is clicked:

    <div id="size-bt