If you get the following…

/Library/Ruby/Gems/1.8/gems/firewatir-1.6.2/lib/firewatir/firefox.rb:1078:in `raise': exception class/object expected (TypeError)
from /Library/Ruby/Gems/1.8/gems/firewatir-1.6.2/lib/firewatir/firefox.rb:1078:in `js_eval'
from /Library/Ruby/Gems/1.8/gems/firewatir-1.6.2/lib/firewatir/firefox.rb:195:in `get_window_number'
from /Library/Ruby/Gems/1.8/gems/firewatir-1.6.2/lib/firewatir/firefox.rb:162:in `initialize'
from /Library/Ruby/Gems/1.8/gems/firewatir-1.6.2/lib/firewatir/firefox.rb:177:in `new'
from /Library/Ruby/Gems/1.8/gems/firewatir-1.6.2/lib/firewatir/firefox.rb:177:in `start'

…the problem is that firefox is already running, probably with a dialog box open. Be sure to close firefox before starting it. The problem is related to this one too…