How to develop a simple Browser Add-on / Plugin using Python?

Can someone here explain how Python can be used in web browsers to control, create or manipulate the content within a user's web browser, or a web-based technology in a simple way?
I have heard that there are tools that convert Python into JavaScript, that can then be run either stand-alone, using technology such as Spidermonkey, Google's V8 engine (for example, using pyv8), or in a web browser. Can someone explain in simple format - how to embed Python in any web browser?