Python is an interpreted scripting language available on a wide variety of platforms, including Linux, Mac OS X, and Microsoft Windows.
Dive Into Python is the best introduction available. It provides good coverage on all the language basics, and common tasks such as web requests and file operations.
For more advanced topics, Text Processing In Python is an excellent reference and tutorial.
Python in Mozilla
Python is also bound to the DOM (starting with Mozilla 1.9). Mozilla chrome (e.g., XUL or trusted HTML) can use Python. The PyDOM page has more information. More information is available on the Python XPCOM page.