That is simply not true.
There is a really simple way to test this. Go to the Acid Test site with Firefox, Chrome, and IE. You'll notice they all handle the tests differently. All microsoft programs may well use the IE rendering engine for displaying HTML, but other programs do not. You can get a plugin for FF that will let it use IE, but by default it does not.
You made my point for me right there. Read the rest of my post:
Your best bet would be to use a browser that utilizes its own execution environment such as Firefox or Chrome, as you can be assured that neither one of them is using Windows kernel hooks to render HTML content.
You assume that I meant HTML content insomuch as web browsing. Perhaps I wasn't clear, but I'm not wrong. Any HTML content outside of a browser, in a Windows environment, is rendered by the IE engine. If you're talking about browsing the web with an actual browser program, then your results will vary.