I'm currently using FireFox. Trying to think of any good add-ons to put on it other than adblocker since I just got it 30 minutes ago.
Downloadhelper allows you to download Youtube videos, among other types of videos.
HTTPS Everywhere and
Ghostery are particularly useful for security and privacy concerns, respectively. HTTPS Everywhere, like the name implies, forces websites to use the HTTPS "protocol", which overlays an SSL/TLS encryption over data you send to a website. Ghostery blocks cookies that track you from website to website and informs you of what you're blocking at any time. Very useful tools.