I am a newbie to much of this scripting and am concerned about all users being able to view my site. I see that most of the DD scripts indicate whether they are IE/NS viewable. But what about compatablilty with Mozilla/FireFox?Well, it depends which Netscape browser we're discussing. NN4 is obsolete, and only some marginal user agents continue to support features such as the layers collection[1]. Scripts that were written for NN4 are unlikely to work on any other user agent.
As for later Netscape versions, most are now based upon the Mozilla Organisation's efforts to develop the Gecko engine. In other words, how things render and behave in Mozilla Suite/Firefox is how they'll likely look in Netscape[2].
Is it correct that this the second most common browser?That would depend entirely on your audience. Some sites may receive mostly Mozilla-using visitors, or perhaps Opera users (I know some certainly do). Some may receive but a few. The demographics of any two sites usually differ, so you can't expect the same proportion of visitors. That said, by most accounts Mozilla is the nearest rival to IE.
Something else you might want to worry about is if your site is functional without any client-side scripting support. Again, depending on your audience, you could encounter a significant proportion of visitors that have completely disabled scripting for various reasons. For a small, personal site this may be of little concern. However, larger sites (especially e-commerce) can't ignore even a small percentage as that could add up to tens or hundreds of thousands of visitors that couldn't use the site because of some unnecessary reliance on client-side scripting. That's a massive loss of potential (visitors and revenue).
Mike
[1] That does mean that user agents other than NN4 support the layers collection. Whilst it won't be common, this does mean that detection of this collection does not necessarily imply that the host is NN4. This is why I've said in the past that object inference ("detecting" a browser based on what it implements) is unreliable and should be avoided just like inspection of the user agent identification string.
[2] They might not look exactly the same. Though Netscape is now all but defunct from a user agent perspective, they were always behind the latest Mozilla release. That meant you might have found bugs that didn't exist in more recent Mozilla versions. Netscape are supposed to be releasing a new version, but I don't know what the story is with that at the moment.