Write browser plugin for a personal web application

I want to make a browser plugin, similar to that are available for Twitter. But in my case, the users must be able to submit tweets to my web application.

I want to know which language is used to write such plugins.

For ie: http://www.codeproject.com/KB/shell/ietoolbartutorial.aspx

For Firefox: https://developer.mozilla.org/en/Building_an_Extension

Good luck :-)