Ok, I'll spare you all of the pro firefox propaganda. You all know it is the best browser out there--else you wouldn't be reading this page.
Here are a few of my favorite extentions (plugins):
1) Adblock
An absolute essential for me. Are you sick of all those obnoxious banner ads? Of popups? Of flash ads? Well adblock lets you remove all of them. The interface is great, and it is encredibly customisable. I'm so used to using it that when I use a different computer I and shocked at how much visual polution exists on the web. For those of you with fast computers, you can download adblock lists which will remove 99% of the ads from the web without you having to do anything. Unforunatly, this may cause lag if you're using an older computer. But, no worries, you can just make your own custom rules to block ads on the pages that you visit.
2) Faster Fox
This extention has many options that you can tweak to make firefox even faster. It also has an option to fix the flaw exploited by some advertisers to make popups using flash.
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3) Forgot one other great extention: Stumble Upon. When you first download it you are asked to select your interests. The options range from Cats, to Islam, to online games, and everything in between. Then, whenever you are bored you select a catagory off your list and click the stumble button. It will then take you to a webpage about that subject. From there you decide if you like the page or not and then you rate it. Stumbleupon then compares these ratings to other people's, and then tries to show you pages that it thinks you'd like based on your previous ratings. It contains absolutely no spyware or ads, and they don't (to my knowledge) collect any personal information.
What extentions can't you live without?