Для этой простой задачи можно применить дополнения:
https://addons.mozilla.org/en-US/firefox/addon/dotjs/
dotjs is a Firefox Add-on that executes JavaScript files in `~/.js` based on their filename and the domain of the site you are visiting.
https://addons.mozilla.org/en-US/firefox/addon/simple-script-loader
A very simple add-on for load script file on-the-fly.
Support to load script file when page ready.