Chrome uses a separate render process for each tab[1], so when one crashes it doesn't KO the whole browser. However this also means Chrome has a larger memory footprint, since each tab is taking up the resources of a little browser. Firefox doesn't do this, and so it keeps a smaller memory footprint than Chrome (oh how times change) but the downside is--as you mentioned--the whole browser freezes if one tab fails.

[1] http://www.chromium.org/developers/design-documents/process-...