Nope.
Firefox runs on Linux; I’m using it now.
It doesn’t use IE because IE does not exist on my Linux machine.
The Windows port of firefox would not be completely rewritten to somehow sit on top of IE.
Anyone can download the source code (program in its orginal human-readable form as written by the developers) to the windows version of firefox just to verify this.
Yes, FF operates differently when you use a different operating system.
Just like the earlier poster who said he had a MAC (where we assume he meant an APPLE computer), your environments are different, ergo you get different results.
Let me ask you this, when you downloaded Firefox, did you download the version for Windows or the one for Linux ?