The stuff below is rapidly becoming "old school" and eventually will be deprecated. So please see New WebExtensions Guides and How-tos on MDN | Mozilla Add-ons Blog before moving forward.
Note that since Firefox 48 you can only develop a XUL based extension in the Developer Edition or Nightly and then you need to set xpinstall.signatures.required to false if you are developing and not signing your extension.
- Create a new folder, like C:\Dev\Firefox\TitleExtension
- In the new folder create file called install.rdf and give it your content based on Create the install manifest
- Edit the following in the install.rdf: RDF/Description/em:id, em:minVersion, em:maxVersion, em:name, em:description, em:creator, em:homepageURL
- Locate your profile folder:
CD %AppData%\Mozilla\Firefox\Profiles
and go to the extensions folder insider it
- Create a file in the extensions folder with the filename of the id of your extension, which should be an e-mail address format
- Edit the file so that it contains the first folder but with a trailing slash, like this: C:\Dev\Firefox\TitleExtension\
- Start Firefox and when prompted allow the installation
- Create the file chrome.manifest in the extension folder, alongside install.rdf
For my extension I used the following MDN pages: