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 sue Add On's that have been developed by others. Some I can't live without, others I have tried and removed.
There are currently six 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 let's you right-click-and-drag 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 mans 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.
Those are the three I absolutely could not live without. The other three 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 "Google Earth Hacks", by the time I get to the "h" in "hacks", 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.
ColorfulTabs - This one doesn't work properly in Firefox version 22.214.171.124, my tabs do not show up in color, but I have set it to fade all the open tabs except the active tab. I'm hoping someday this one will color my tabs.
What Firefox Add On's do you use? Are there any you can't live without? Since there are thousands of Add On's, it would be interesting to see if anybody here uses a *Must-Have* that I've never known about.