We use FireFox 3.0 extensively and havent had any real problems with it that I am aware of. We even use it in an Ajax environment, where we dynamically add it to a page that didnt have it after the fact and that works in FireFox 3 too.
Our biggest gripe is the drop down to text box in Safari.
You can see our usage of FreeTextBox in FireFox 3 in our support forums at http://bizmall.ph