This is because security in the browser is paramount. Everyone including Firefox, Google, Microsoft and Apple all realize browsers have to be kept up to date. Microsoft has even created a site begging people still using IE6 to upgrade. Chip says he liked the separate search boxes in the older browsers, but Leo says it's not worth the security risk of staying with an older version. Firefox also had a severe problem with memory leaking causing a crash, so updating will fix that too.
Yes, Firefox is fine. Awhile back it had a memory leak, but that has since been fixed. It can, however, get bogged down by add-ons, and it isn't as secure as Chrome. Firefox is also pushing Enrique to upgrade to version 12, but he thinks it's slow. Leo says he really has to keep up to date with browsers because of security concerns. The only reason Enrique didn't want to switch to Chrome is because of the add-ons he uses, but there are similar add-ons that he can use with Chrome.