If you've been targeting your applications at MS Internet Explorer, you've probably been getting requests to support Firefore 3 since it's got over 8 million downloads (which hopefully translates into 8 million users) worldwide.
When starting off with web development for Firefox 3, you'll probably want to get the Firebug and Web Developer add-ons. They have a different feature set than the Internet Explorer Web Development toolbar so you'll probably have moments of doh-I-had-it-in-IE or cool!-IE-didnt-have-this.
A couple of commonly reported issues by web developers moving from IE to FF3 are:
(i) the inability to resize the file input form control
(ii) the use of the textContent instead of the innerText property.
(iii) use of document.getElementById instead of directly referencing the element by it's ID
(iv) really frustrated IE web developers cursing the web standards