Possible advices:

a) use a non JS solution, like WWW::Mechanize or Selenium.

b) use an automation addon for Firefox like iMacros.

c) put your approach in a frameset, one frame holding the JS the other the page to load. (but all parts from the same domain!)

UPDATE:

d) it may be that you can load a HTML page into the sidebar which starts your testscript on load. (UNTESTED)

e) if it's only about loading pages into Firefox you can also write a simple perl script using firefox -remote (IIRC linux only) or mozrepl for communication.

f) last but not least WWW::Mechanize::Firefox