I love Foxmarks, but it seems to waste a good bit of CPU time.
Strange, I've not had any problems with it at all, across an whole range of machines and Windows versions.
I have had almost nothing but problems with IE Tab. It almost works, but then things just fail for mysterious reasons. Virtually everything I need IE for fails to work quite right in IE Tab.
I've thrown loads of odd stuff at IE tab over the last few years, ActiveX heavy online banking sites and lots of IE only Intranet apps. I can't remember anything that didn't work just as well in IE tab as it did in IE.