User scripts are add-on programs that can perform a wide range of actions within web pages. Most scripts are written by individuals or other companies, not Mozilla or Google. The main hosting sites keep an eye out for dangerous scripts, but you should only install scripts you feel comfortable with after considering their potential value to you and whether the author looks trustworthy. It's not always an easy decision!
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The scripts are hosted on the following sites:
To learn more about Google Hit Hider by Domain, click here.