Most browsers allow extensions for them. But, as I understand, there separate extensions needed for different browsers.
Is there any way to write one extension for all browsers? Can such thing be created Java, .NET, or Python?
In my opinion you can use Crossrider, with Crossrider you will write one JavaScript code, that will produce an Chrome,Firefox and Internet Explorer compatible extension.
Crossrider have a powerful API that hide all complexity while writing cross-browser code.
Try to give it a shot in: http://www.crossrider.com
Disclaimer: I worked at Crossrider