I've been having trouble for a long time with Firefox being damn slow to open new windows. I finally found the fix. Firefox hangs - MozillaZine Knowledge Base.
User Agent Switcher
Reinstallation or updating User Agent Switcher extension can cause hangs. Start Firefox in safe mode, uninstall the User Agent Switcher extension, and restart Firefox. You may now install the extension again without it hanging.
Instant improvement. Windows pop open in Firefox 1.5 under Mac OS 10.3 and 10.4. Not as fast as Safari but not bad at all.