Why would anyone use firefox which is 20 times slower than Safari and Chrome even if you're on, I don't know, 1TB internet connection. Sarafi+Glims kick some serious Firefox ass, if you need add-ons. But if you prefer clean and fast browsing use Chrome.
Someone also said they use VLC, well, I (and most people used to normal user-oriented interface most "developed for Mac" apps use (Firefox is not in this category)), hate it, and use QT10+Perian which can read all formats VLC can read except for MKV. Until the new Perian comes out at least. Another handy app you might want to consider is AppZapper for "uninstalling" apps (i.e. deleting preference panes and registries most applications leave behind when you trash them), Acquisition for P2P, Toast for burning media, RarMachine for .rar files and Transmit for FTP. Also, another handy freeware which lets you log out without having to quit all your running applications is MacLoc.