[EidolonAH]

Firefox is a bit of a dog these days, too much bloated crap is my thoughts to be honest. Pretty much all of my Themes are image heavy, you cannot make a Theme look this way without it being as such. Saying that, there are a couple of things you can do to lessen the 'lag' effect, first and foremost, FF's cache is a real resource hog, clearing their cache from time to time will help, every time they visit a new page FF stores everything on their PC like all browsers, for some odd reason FF just seems to be a real dog at this. The very latest version of FF is seriously bugged, I found it causing far too many problems on established sites, so I have rolled back to the previous version myself. The developers of FF don't give a damn about forum solutions, they even have the cheek to deny some bugs even exist.
Secondly, I save all images for Themes at the highest pixel rate to maintain pixel integrity, you can open all of the image files in something like photoshop and reduce the pixel quality. This will make the Theme less load intensive but of course it will no-longer look the same, the images used will look hazy and blotchy, the more pixel quality reduction used the uglier they will look.