html - PNG image is not loading properly in Firefox while loading the ascx page via JavaScript -

i loading ascx file using javascript. when ascx loads first time, background image of window displays properly. when load second time, background image not visible. when hover on section, again visible.

the issue occurs in firefox. works fine in chrome.

my html:

<tr class="adminpopupbggrey ">   <td colspan="2" class="adminpopuplines">&nbsp;   </td> </tr> 

css:

.adminpopuplines {     background-image: url(/img/icons/adminpopuplines.png);     background-repeat: repeat-x;     background-position: center;     height: 2px;     width: 4px; }