- Pop-up blocking is better implemented on Mozilla ( AOL lives from ads, and NS "belonged" to AOL)
- AIM, etc... is not installed on Mozilla, so if you want a browser only, go for it.
Consider this: When a Mozilla release is almost ready, it is taken by NS developers and customized to fit their "commercial" principles, so I guess Mozilla has less possibility to become unstable.
If you are not using Composer, mail and /or debuggers, go for Firebird instead of Mozilla. It works "better" than mozilla.