I want to write my app for Firefox OS. What kind of languages (like Java for Android and Objective C++ for iOS) and tools (like Eclipse, Xcode) to use?
Apps for Firefox OS (previously known as Boot to Gecko) are developed using HTML 5 and Javascript.
References:
http://www.mozilla.org/en-US/b2g/
https://developer.mozilla.org/en/Mozilla/Boot_to_Gecko/
You can use any web development tool you like. Firefox has a lot of built-in tools to check out: https://marketplace.mozilla.org/developers/docs/devtools The responsive mode tool is very useful for Firefox OS / mobile apps.
Besides tools, the most important skills to learn are techniques for building web apps: https://marketplace.mozilla.org/developers/docs/references