List-it is a fast, simple, 100% Free+Open Source (MIT License) note-taking tool for Firefox. Designed by a research group at MIT, List-it lets you quickly take short notes to yourself + quickly access them whenever and wherever you need ‘em.
Here is a little more about this add-on from the developers:
list-it is a simple, free and fully open-source Firefox-based note taking tool to help you manage short important notes to self. It is designed to be simple, fast and minimal so that it is easy to use and understand. We are also constantly finding and improving ways to help you find your notes again so your information scraps don’t get lost.
All notes are kept on your computer in your Firefox profile. You can back up your notes to our server hosted at MIT.
List-it can also be used as an tool for human computer interface researchers to study the in-situ use of personal note taking tools. It is instrumented to provide high fidelity activity logging and aggregation/uploading of use data, which can be enabled if desired. These fine-grained timestamped logs of when and where actions were taken (note creation, edit, and access) so can then be studied later to understand note taking practice.
Want to learn more? You can pick up List-it from the Firefox Add-ons website.