I was wondering if it\'s possible to write Firefox extension using .Net Framework? Had anybody such experience in writing Firefox extensions using C# programming language? A
Assuming you're asking about writing an extension (and not a plugin), the best place to get started is with the info at the Mozilla Dev Center.
Konamiman is correct: extensions are mostly XUL (an XML grammar) and Javascript, packaged into a ZIP file with the extension .xpi. The various links from the page I linked should answer basic questions, like the one you posted above:
https://developer.mozilla.org/en/Building_an_Extension
https://developer.mozilla.org/en/Extension_Frequently_Asked_Questions