Strange behavior when resizing Firefox window

[Zfennn]

I have 2 problems which seem to appear in Firefox only. When I resize the Firefox window to a width lower than 750px the image becomes ridiculously huge (width exeeding 8 mio px in width). Second problem is that in Firefox ul class="slides" style="height: 90px" is set when I reload the page from server. Seems like ImgHeight/ImgWidth are not set correctly because the image size might not yet be available when reloading the page. When I load the page from browser cache it works fine and sets the correct height. But even then the size of the image gets to big when I resize the Firefox window. It's Firefox version 62.0. Any ideas why this happens?