Extensions are not plugins. Take a look in your Firefox addons menu if you don't believe me. Plugins are things like Flash, they're written to a specification (NPAPI) that originates in the Netscape days, it is supported by all major browser makers EXCEPT Microsoft. That's why on Windows there's two versions of Flash (not talking 32/64 bit issues here) one for IE and one for everyone else.
IE used to support the Netscape plugins API, but removed it around about IE5.5 if I remember, the idea was to force developers to write an IE version (ActiveX) if they wanted to support IE. They were probably hoping that plugin developers would have just developed for the more popular IE and ignore Netscape, finally killing browser competition off for good.
Fortunately the increase in popularity of alternative vouchers has kept the NPAPI alive, meaning that plugins written for one browser will work on them all.
Now the problem with plugins is they're written in compiled code and therefore a version needs to be written for each OS. Extensions on the other hand are usually written in XUL and JavaScript and so extensions will normally work on any platform, but extensions are specific to a particular browser. So plugins are OS specific, extensions are browser specific.
So when I read that IE8.1 supports Firefox plugins, my first thought was that IE was bringing back support for the NPAPI that they removed in IE5.5 making it easier for plugin developers. I knew that it was not possible that IE could support Firefox extensions. That would be almost impossible to implement for anything more than the most trivial extension.