When you're using IE Tab to load a webpage it is actually using Internet Explorer inside a Firefox 'frame,' so all of the IE vulnerabilities apply. In addition, none of your Firefox extensions or settings have any effect on the webpage. Thus, despite loading it in Firefox you should think of it as loading the website in IE and not even running Firefox.
Hope this helps,
I sometimes need to use Internet Explorer for some webpages so I installed the IE TAB.
I would like to know if you still have the security of the Firefox Browser with this ADD ON.