(you're on the web design forum now, you know) you don't use Firefox or IE to make your application. Those are browsers only.
But you surely check your application works correctly on current versions of both, that is at least IE7, IE8, Firefox 3.0, Firefox 3.5. Any serious application should be tested also against IE6, Firefox 2, Opera, Chrome and Safari (both the Windows and the Apple variety).
which is the best one to use IE or Firefox ?.
My friends did suggested me for Opera, but they've not yet checked the Opera developer tools. Please suggest.. I want your valuable answers.