Firefox allows extra customization. There are themes available for download and creation. In addition, there are hundreds of add-ons that users can download.
I currently use Down Them All! which is a download manager, Adblock Plus, which blocks picture-based advertisements, and anything else that you choose. For my non-picture ads, I have a program called FlashBlocker. I also have a password keeper, which stores passwords for nearly every site. In addition, I have an e-mail checker, which notifies me of e-mails on multiple accounts, within 10 minutes. There are also greasemonkey scripts available, which range from everything. The main one I use is DownloadHelper, which allows you to download videos from suitable sites, such as YouTube. Finally, I have weatherbug installed on my computer.