Add your custom javascript to a site
<p>This is a question that is not easy to express... I hope someone understands it ;)</p>

<p>I would like to add some javascript to a forum that I visit every day. Simply add some functionality. I tried to google this, I know it's possible, but couldn't find anything. So I came here to ask: What is the term or name of the process of adding frontend functionality to a site from you browser?</p>

<p>I don't care which browser, whichever is easiest to do such a thing as adding custom javascript to a site.</p>

<p>Thanks</p>
