mine is productivity combined with issue tracking [i.e. issue tracking combined with effective planning methology] (issuesdone.com)
Nice feature you have is keyboard navigation, I think that's a must have for web apps.
The reason behind requiring Firefox or Safari is simple enough too. We have not enough time yet to support every (even popular) browser. We'd like to spend time doing essential things, not tweaking HTML and CSS these days. Again, early adopters are those who are relly interested in using our app at the early stages and they seems to have no problem using FF or Safari to access our application.