To create a firefox addon:

1. Create a zip archive which contains the following:
content (a directory)
bootstrap.js
chrome.manifest
icon.png
install.rdf
README

e.g. in Linux you can "Select All" then right-click and choose Compress.

2. Rename the zip extension to .xpi

3. Drag and drop the .xpi into Firefox


To create a Chrome extension:

1. go to chrome://extensions and Enable "Developer mode"

2. Click "Load unpacked extension" and point to the parent folder 
(inside of which this README file is)