I'm planning to develop a simple Firefox extension that will shorten URL of a currently active tab, display a popup with the shortened URL, and place it into the clipboard.
However, after searching around for a good Firefox extensions tutorial, the most officially-looking links I found are:
It's 2013, isn't there an easier way of building Firefox extensions?
Yes there is!
The links you provided in the question are unbelievably outdated. There is a new, much better way of developing Firefox extensions -- Firefox Add-on SDK.
However it's pretty hard to stumble upon it by just googling along the lines of 'firefox addon tutorial'. I'm amazed Mozilla doesn't advertise it more aggressively, or at least mention it on those pages you found.
Steps to get started (Mac/Linux, but should be pretty similar for PC):
source bin/activatefrom the SDK dir (the same dir the README file is in).
cfx docs-- this bootstraps local copy of SDK docs and opens it in your browser.
cfx initinside the extensions dir -- this generates all the necessary files/directories.
lib/main.jswith just a few lines of JS to place a custom widget onto add-on bar.
cfx run-- this opens fresh Firefox instance with your new shiny extension in it.
There is a new standard, called WebExtensions
There are currently several toolsets for developing Firefox add-ons, but WebExtensions will become the standard by the end of 2017.
If you are writing a new add-on, we recommend that you write a WebExtension.