Add-ons (extensions) are the reason I use Firefox. There are times when I got tempted by other browsers (Safari, IE7, Chrome), but I always return to Firefox. These are my top 5 must have Firefox add-ons.
1. Adblock Plus
I understand that nothing comes free, and many "free" content on the net are supported by ads. Advertising is one thing, but I despise ads that are super intrusive. Blinking banners and long-loading-crash-prone-flash based ads are so common place now that they disrupt my internet activity. Not only that, they're using up my bandwidth and slowing down my browser. Enter Adblock. Right click on the annoying ad, and you can set up a wild-card filtering, and that ad (and all ads related to the wild-card criteria) won't show up anymore, forever. Of course, doing this one by one can be tedious, considering there are sites where there are too many banner ads around. Enter Adblock Plus. You can subscribe a list of pre-determined wild cards and voila, no more annoying ads. If there are legit elements that got blocked, you can simply whitelist them.
A must have for anybody that are concern about security. There are too many web scripting exploits out there, and blocking one by one is tedious, if not impossible. NoScript will disable every scripting (java, flash, etc). Then, you can whitelist the webpages that you trust. In the beginning, it's somewhat annoying whitelisting websites since almost any websites now use some kind of scripting. Very useful when you're browsing questionable websites and for blocking sites that use script to plant tracking cookies. Coupled with Adblock Plus, you will experience a lean and safe web browsing environment.
Very useful if you have multiple PCs using Firefox. Foxmark will sync your Firefox bookmark to the cloud. When you have a new installation of Firefox on a new PC, simply install and log-in to Foxmark, and your bookmarks will downloaded and synced. I used to use Google's browser sync, which have more syncing features than just bookmarks. Alas, Google abandoned the project when Firefox hit 3.0.
Let's say there's a part of a website that you want to save, or you just bought something online and want to save the receipt. Saving the HTML sometimes is a hit & miss, especially on SSL pages. Printing the page is a waste of paper. Enter Screengrab. With Screengrab, you can save a part or the complete page of a website as an image file. No worries of different HTML rendering or wasted paper. Extremely useful!
Recommended by a Yan Dao. Ever browse a Japanese website, and wonder what is the meaning of certain words, but don't want to keep copying/pasting them to a translator? Install Rikaichan, and the Japanese dictionary. When enabled, it will automatically show the possible translations of the word that your mouse pointer is on.
Let's compare the other browsers. I don't see any capabilities of the above possible on IE7 or Chrome (not yet). There is Adblock for Safari, and you can sync bookmarks using MobileMe (not free). Still, I find Firefox to be the most flexible. Firefox maybe slow at times compared to the other browsers, but the add-ons are worth it.