I've been a Mozilla and more recently Firefox user for a long time, but
only recently noticed the availability of extensions. (I know :") )  I'm
really excited by the extensibility, but cautious about the tie-in to a
single browser development environment.  I know Microsoft ignores this
all the time, and unfortunately I'm finding more and more sites that
seem to *require* IE to use.  My question is, even though the underlying
XML is "open" and "standard", doesn't the use of XUL lead to the same
sort of "lock out" for users of typical "public" services, such as
libraries who frequently have no choice (or even knowledge) of which
browser to use?

Is there anything that can be done, or has been done, to take these
great extensions, and the tools to create them and make them coexist
with or "embrace and extend" the IE universe?