I'm one of the many Firefox fans around the world, I use Firefox for 99% of my web browsing. One of Firefox's great benefits is the ability to use Add-On's that have been developed by others. Some I can't live without, others I have tried and removed.
There are currently eight Add-On's in my browser:
Mouse Gestures - If for some reason I was only allowed one Add On, this would be the one. This tool lets you right-click-and-drag (gesture) to go backwards or forwards (previous page or next page). You can also set it to go up, down or sideways and you can set what your browser does with each gesture. I found a new one today - when I gesture "Up", it takes me to the top of the page. This is great for long pages.
IE Tab - Yep, there are still some pages that work better in Internet Explorer. The IE Tab puts a little icon in the bottom right of your browser, click it once to render the page in IE within Firefox.
McAfee SiteAdvisor - This also lives in the lower right corner of my browser and is usually green. If it's yellow, that means be leery. If it's red, that means the site has proven to have malware or is known for a lot of spam. I have seen false readings, the Texas Tech University Health Sciences Center is a very legit site, but McAfee flags it as red. And it's not a true add on, but an application that runs with Firefox (and IE).
Those are the three I absolutely could not live without. The others I have are:
CustomizeGoogle - You've got to install it to understand, but your Google search box changes to a drop down box, so if I'm searching for "AMD vs. Intel", by the time I get to the "I" in "Intel", the whole phrase is an option to click on.
FoxClocks - This one also lives in the lower right corner. Hovering over the icon gives a pop-up with a list of the current time for every location I want. I have a good friend in Greece and we IM on occasion. I can hover and instantly tell what time it is in Greece, no need to open MS Messenger if it's 3:00 AM.
Linkification - When a webpage has links in cleartext, i.e. the link is spelled out but it isn't a hotlink, Linkification makes it a clickable link.
WellRounded - Gives the url and search boxes rounded corners, makes them look nicer. Just eye candy.
Tab Effect - When switching from one tab to another, it "rotates" as if each tab was the side of a cube. "Pimp My Firefox" is how one user reviewed it. This one might become annoying after a while, it does add a second or so to each tab switch. But even if I end up turning it off (probably will), I will definitely leave it available and turn it on if someone is going to visit and we will be using a browser - the "Whoa - That's cool!" comments will be worth it. And it absolutely goes on my laptop for when I give presentations just to add some "Wow" factor.
Fast Dial - This turns my "About:Blank" page into 12 thumbnails. Very handy.