Paul,

I just spent a couple of hours fiddling with this, and it's entirely a browser issue. Nothing really works other than feeding unique URLs to Firefox each time. So, Hijack v470 is now available, with support for a "IGNORE=xxxxx" parameter that you can use in the URLs. Eg. Code:

document.images.image1.src = 'http://my.empeg/proc/empeg_screen.png?IGNORE=' + Date();

Just for kicks, I also updated the HTTP headers returned by Hijack to include more of the modern "not cacheable, you idiot!" keywords, but none of them really have any effect here.