yes there are other browsers but their marketshare is small enough to make it not worth the investment making sure your code will work for them. Mozilla SHOULD be fully compatible with Netscape as both share the same engine. IE has over 90% of the market cornered, Netscape/Mozilla another 7% or so. Of the remaining 2-3% roughly half is covered by Opera. That leaves just 1% of the market for all other browsers combined. Unless you're specifically targetting those people (GSM phones and PDAs for example) it is not economically feasible to write code for them. In fact, it may not be feasible to write code to ensure compatibility with anything but IE! What I do is use only W3C standard code and check if it works in IE. If it does, it goes life and any browser that doesn't support it has a problem I don't care about (had they been standards compliant the code would have worked). Of course if you're coding for an intranet where the company dictates Netscape as a browser for 50% of the machines and IE for the rest (I've seen it, different departments used different browsers due to corporate politics) the situation is different.
An attitude, in my opinion, that helps ensure IE's domination of the market for years to come. Of course, one has to make sure that things work in IE due its market dominance, but just coding to W3C standards usually won't accomplish that since MS has shown no interest in improving IE's W3C compatibility. As a conscientious web developer interested in improving the web, I make sure that my web apps work in all W3C compliant browsers (Gecko-based browsers, Opera and Safari being my principle targets), and then do all the gritty work to make things work in IE as well.