The default XSLT processor for IE6 is MSXML 3.0. You may forget about running XSLT in MSXML 3. What it supports is not even a subset of XSLT, but an older project. However, most people will have MSXML 4 or higher installed either through a software update, or in the ms office, or in some other way. XSLT in MSXML 4 and above is very fast, much faster than XSLT in Firefox.
So, do not check the version of IE, but the version of MSXML. Try MSXML 6 first, and if it does not exist, rollback to MSXML 4. Do not try to use MSXML 5, it was an intermediate version used by Office, but is no longer supported by Microsoft.
MSXML supports extension functions (Firefox is not sad), so you can add EXSLT support using javascript.
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