Yah's Link Checker for Firefox requires that you accept the following End-User License Agreement before installation can proceed:
Copyright (C) 2010 yahavatar - yahavatar.com
This program is free software; you can redistribute it freely and modify according to your own purposes, however, a distribution of such modified version is NOT permitted. Using portions of this code is strictly prohibited for anything beyond personal and/or educational purposes. Any redistribution MUST include all files and links included in the original package as well as reference to the author and/or any contributors.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
In the end, your Mileage May Vary, (YMMV), very much depending on the pages you are browsing, the speed of your internet connection, every leg of the journey from your browser to the filehost(s), and the response time of the filehost(s) themselves. Whew! Generally you'll find this Add-On extremely useful, but on occasion, you won't.