Make sure the extension crashes the browser even if just once a week. What will your fellow developers say of you if you even can’t crash the environment with pure JavaScript? It’s sometimes enough if the extension simply doesn’t work.

Stuff everything — and I mean everything — inside the right click context menu. Or, in fact you can add your menu items anywhere as long as it’s not that convenient for the user. Forget about nice buttons the user could move around in the toolbar.

Keep the usual GUI design rules in mind. For example, if your feature is called, say, “dTaOneClick“, make sure the user can’t really just click once but maybe twice. No keyboard shortcuts allowed.

Forget about other extensions. And stuff even more things inside the context menu with useless descriptions. Do not group your menu stuff inside a submenu.

Never continue supporting your extension. After all, every Firefox user wants to install Nightly Tester Tools to be able to use your extension after a critical update from 2.0.0.3 to 2.0.0.4.