How to include JavaScript on a website through a Firefox addon?

How is it possible to execute JavaScript code on a specific page through a Firefox addon. I know I can use Greasemonkey but since I'm porting one of my Chrome extension, I want it to have the exact same features. So my question is, is it possible to execute JavaScript through a Firefox addon as if that JavaScript was executed in the page itself.

I was able to work it out somehow but every time I open a new tab, it gets executed in that tab again. Someone please help me fix this pr开发者_StackOverflowoblem. I also want to use jQuery with this.

I use firebug, and go through the console to manually execute JS functions for testing. Alternatively, if you have to insert a whole JS include file, check out Fiddler.