I wouldn't expect this to work either. Even though you've wrapped the launching of Firefox in a script it isn't truly a script in the sense that it can be run during startup of the system. For example, in order for Firefox to launch it would need X running and access to open a GUI on a particular session of X.
I would attack the problem in a slightly different way using one of the methods discussed below. In particular I typically use the alternative approach.
Usually you put X related items that you'd like to run in the context of the user (you) in one of 2 files:
Some commands such as
xset may not work from within these files. If you'd still like to automate something like this you may be able to add it as a "Startup Application".
Create a shell script,
xset r rate 120 66
Add a startup task to run the shell script.
Open the application Startup Applications, under System → Preferences
Then click the Add button
Once you're done OK & Close the dialogs and to test it out logout and log back in.