Couldn't find a good link earlier, guess I didn't have quite the right keywords for search.

Here we go, looks like its 17 years old now:

https://bugzilla.mozilla.org/show_bug.cgi?id=98168#c99

This bug is specifically about <xsl:text disable-output-escaping="yes"> not working in Firefox. How is disabling output escaping relevant in regards to sharing templates between pages?

from the linked thread:

> The only combination that fails to render these entities correctly is Firefox/XSLT.

Which is one good reason not to adopt XSLT to implement HTML includes. You just don't know what snags you'll hit upon but you can be sure you'll be on your own.

> Bug 98168 (doe) Opened 24 years ago Updated 21 days ago

Well it does look like someone's still mulling over whether and how to fix it... 24 years later...