My Firefox add-on won't show when I install it - firefox-addon

Question

So, I'm utterly and completely new to making Firefox add-ons. I have been trying to make a really, really simple one which will run some JavaScript on the page when I click it. That's literally all I want it to do -_-
However, it will install when I drag it into FF/Nightly, but then not show up in the customise menu. It will say it's installed, but that's it. (I also can't open its preferences, but, I'm really not complaining - I just want a really basic add-on).
If you need the code/entire XPI itself, I'm happy to supply. Thank you :)
I am using the following directories and files:
.
├── chrome
│   ├── chrome.manifest
│   ├── content
│   │   ├── boustrophedonise.js
│   │   ├── browser.xul
│   │   └── prefrences.xul
│   ├── locale
│   │   └── en-UK
│   │   └── translations.dtd
│   └── skin
│   ├── icon.png
│   └── skin.css
├── defaults
│   └── preferences
│   └── pref.js
└── install.rdf

Answer 1

While there may be other problems, your chrome.manifest file is in the wrong directory. It needs to be in the root directory (the same directory as the install.rdf file). This is covered in both the "Setting Up the Development Environment" and "Create a Chrome Manifest" sections of the Building an extension tutorial on MDN. The first section explains that you should start with a directory structure that looks like:
.
├── chrome
│ └── content
│ └── sample.xul (Much of your content in this directory)
├── chrome.manifest
└── install.rdf
The chrome.manifest file is one of the required files for a Firefox add-on. It defines what URLs like chome://myExtension/content/* point to. In your case, without the chrome.manifest file being in the root directory, the URL in your install.rdf of chrome://boustrophedonise/content/options.xul is invalid and is probably complained about in the Firefox Browser Console when the extension is installed (or perhaps when the add-on manager is displayed, or the link for your add-on options is clicked). Firefox probably also complains about the lack of a chrome.manifest file. Obviously, since Firefox is unable to find the chrome.manifest file it does not apply the overlay onto chrome://browser/content/browser.xul, nor the skin, both of which are defined in that file, which are the things that hook your add-on into Firefox.
So in your case, you need to move the chrome.manifest file such that it is located as:
.
├── chrome
│   ├── content
│   │   ├── boustrophedonise.js
│   │   ├── browser.xul
│   │   └── prefrences.xul
│   ├── locale
│   │   └── en-UK
│   │   └── translations.dtd
│   └── skin
│   ├── icon.png
│   └── skin.css
├── chrome.manifest
├── defaults
│   └── preferences
│   └── pref.js
└── install.rdf