Inject JavaScript into web page

I'm planning to build a program to automatize some tasks on my computer, in specific I want my app to: - gets the first line of text from a texfield - puts that text into a URL (for example www.website.com/text) - opens that URL - injects a JavaScript code into that web page (I already have the JavaScript code created) - after injecting the JavaScript into the web page, it goes back to the textfield and gets the second line of text - and continues this way until the last line of text.

I was planning to build a Firefox WebExtension. Someone can help?

Thanks in advance

Angelo G.