web images reload when switching to other tab for a while and then switch back

I made a website but find an issue, when I switch to another tab on Firefox for like 5 minutes and then switch back to my page, I notice that all the images in the index.php and in the navbar(header) are reloaded, this doesn't happen to other websites online, so I am assuming it is not an issue of the browser, however, when I tried my site on IE, there is no such an issue. So, what may cause the problem? The website contains html, css, php and some javascript.

javascript/jquery is for the slider, ajax stuff and change button value, didn't find anything to do with the issue.

Will some php/css code cause the issue?

I added to the head, but it doesn't help.

Thanks!