Web designers will rejoice at the opportunity of building apps exclusively for the Firefox OS phones using HTML5 and Responsive Design. This is a brand new market, and developers in Spain will have an early entry.
Next steps for the new mobile product will be Poland, Colombia and Venezuela, which have upcoming launches soon. More countries to follow.
The Firefox mobile OS devices currently can only be purchased in the markets where they are for sale, which discourages web developers from other regions.
For testing purposes, the Firefox OS Simulator is an option to test open web app projects--although it is in an early stage of development and still needs some work to improve reliability. The Firefox OS Simulator is a developer tool that enables creators to test and debug Firefox OS apps on the desktop.
SOURCE: https://developer.mozilla.org/en-US/docs/Tools/Firefox_OS_Simulator