Page Not Found in Python with Numpy

Eeeek! It sounds like you must have also deleted the notebook file, while you were deleting FireFox. Fortunately there is a way to recover a clean notebook, but any work you have done previously is probably lost. Well, unless you accidentally renamed the notebook, instead of deleting it. Please have a look at this article, which is also linked from the DLS FAQ Thread (worth a look on general principles).