If you are persistent enough you can find a way to run Perl scripts in Firefox. I've used Javascript + XPCOM to open sockets to a Perl RPC service I created which will allow Javascript to make calls to Perl subroutines. I used it to wrap (partially) DBI methods so I could talk to real databases from Javascript. I also utilized a <perl> tag to embed Perl in the XUL. The RPC client (written in Javascript) has a method that will extract the Perl code on page load and send it to perl to be executed. The technique does not allow Perl to replace Javascript, but allows Javascript to take advantage of some of Perl's strengths.