Lyne, why are you opening a new instance of the browser?  Open a new tab instead.  Opening a whole new instance of IE uses a lot more resources than opening another tab in the existing instance.  I use Firefox and have two tabs with Renderosity right now, and often do that with this and other sites, for the same reasons you gave...I would have my account in one tab and a product search in another tab, things like that. 

I just tried IE8 and opened two tabs on Renderosity and there was no problem. 

I don't know what script or plug-in you use to change the background color, but it might be that it is incompatible with something on the site.  Or, as I said, it could be that opening two tabs would get you better results.  If you use too many resources, you can start to have video problems.  If I have too many tabs and too many other applications open, and open one more tab, sometimes the video goes nuts.  This happens a lot on the ShareCG site.  I think it's because they have so many graphics and links on a single page, but I'm not sure.

Dana