Ice Weasel is Firefox minus the Firefox brand (name, logo etc. etc. cannot be used without Mozilla's explicit permission). The “use a new process per tab” probably means that you visit sites which use Flash/Java/etc.: the idea is that these plugins run in their own process so if a flash or java program crashes (which especially flash programs are wont to do as the flashplugin is not quite as stable as one would like...) the browser is otherwise unaffected.
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