That’s the question.
Which of them do you use and how satisfied are you.
I use NoScript, and I’m very satisfied with it. It allows you to easily tailor which sites to allow or forbid scripting upon, as well as allowing you allow temporarily by session.
It’s swell.
Adblock plus. It’s pretty amazing. Actually, what’s amazing is when I have to use a computer that only has IE; I can’t help thinking “what the hell are all these…ohhh…riiiight”
“Download Them All”
You can break up file downloads and resume when you want to. You can set the filter and in one click get all of a type of files qued for download. It works very well.
SessionSaver saves all the tabs you had open the last time you closed Firefox. It seriously changed the way I browse. I no longer have to worry about losing all my tabs when I reboot.
IE View adds a context (right-click) menu item to open uncooperative pages in IE. You can also configure it to always open certain pages (Windows Update, etc.) in IE.
Tabbrowser Preferences lets you customize many aspects of tabbed browsing.
Adblock Filterset.G Updater keeps your block list current automatically.
Flashblock, DictionarySearch, Bugmenot and the previously mentioned Adblock and Ablock Filterset.G
Be careful about Flashblock – if you’re having memory leak troubles, it’s one of the possible culprits.
That is awesome! Link?
You can thank me later 
If you have a Gmail account, Gmail Notifier is a very handy wee thing: a red envelope that sits at the bottom of your browser window, and turns blue when you have new mail, with the number of unread mails you have in your inbox. It’s quite tweakable, with an intuitive control panel.
I prefer IE Tab, which uses the IE engine within Firefox, with a single-click reload of a page using IE.
Foxpose lets you see thumbnails of all the tabs you’ve got open at once.
Colorful Tabs makes your browser tabs appear in different colors.
Download Statusbar eliminates that separate Downloads window in favor of a statusbar at the bottom.
Paste Quote is an essential for SDMB users–it pastes whatever is in the clipboard with quote tags around it.
Sage is an excellent RSS/atom aggregator.
Performancing is a nice blogging client that I use both for my LJ and my blog (cough).
All-In-One gestures is sweet. The one I use most is right mouse + drag to embiggen* the text, since I often browse the blogs or the SDMB on my laptop while I sit back on the couch.
Gmail Manager is (or at least was) a bit more tweakable than Gmail Notifier, primarily in that it can check multiple accounts.
Image Zoom delivers what it promises.
(All of these are available from the Firefox Extensions page; sorry for the lack of links.)
- It’s a perfectly cromulent word.
Here are the extensions I have installed and find most useful:
Adblock- Allows the user to block banner ads and other advertising images on the page.
Adblock Filterset.G Updater - This updates your adblock list with a set of filters that blocks most ads.
Colorful Tabs - this makes each tab a different color and easier to differentiate at a glance. Helpful if you normally have many tabs open.
PDF download - this allows you to choose whether you want to view or download a pdf when you click on it. Comes in handy when you click on a link you don’t realize is a pdf, it doesn’t automatically download.
FireFTP - this is a handy FTP utility that you can open in a tab within firefox
Tabbrower Preferences - this includes functionality for tabs including the ability to reorganize your tabs by dragging and dropping
Gmail notifier - very handy, gives a notification in the status bar when you get mail
ColorZilla - this is great for those who do web or graphic design. It gives you a color picker in the status bar where you can click on any part of a webpage to get a reading on the color of that pixel. You can also double-click to get a colorwheel.
ForecastFox Enhanced - This is what weatherbug should have been. You get a little doppler radar and icons for current and tomorrow’s forecast. Very handy now that hurricane season has started.
Del.icio.us - This gives me shortcut icons for tagging sites to my del.icio.us account without having to navigate away from the page.
Greasemonkey - greasemonkey is an entire new set of ‘add-ons’ unto itself. It allows you to download user scripts that let you determine how you interact with the sites you visit. This is really a very significant advantage of Firefox. For example, a user script could be used to rearrange the order of the information on a site, to present it in the way you want to see it. Another example of what a user script can do is present a comparison box when shopping, similar to what amazon does natively. User scripts can be found at http://userscripts.org
Fasterfox - this is supposed to help optimize page loads, and includes a page load timer and some options for adjusting settings for caching links.
Search Engine Ordering - this allows you to change the order or remove the search engines listed in the upper-right hand corner
IE Tab - this allows you to view a web page in IE embedded within a tab. This one extension alone is the answer to many people’s reluctance to switch over to Firefox because ‘there’s always that one site that will only view in IE’. Well, now you don’t even have to load another browser.
Does anyone know of a good spell check extention? I had one before, it was fantastic, I’d just click on it and it would check the spelling of everything in the field that I was typing in, but it got nerfed 2 updates ago and I haven’t been able to find a new spell check extention that I can make work.
Text size toolbar - Adds text size buttons to the toolbar.
Search IMDB - Searches IMDb for highlighted text(movie titles, names, etc.) via right-click menu.
Groowe Search Toolbar - Groowe Search Toolbar bundles toolbar functions for more than 20 search engines, shopping and download sites including Google toolbar, Yahoo, Ask Jeeves, Teoma, Amazon, Download.com and others. The entire toolbar reconfigures when you select a different engine and it includes many advanced features found in each engine. You can also easily repeat your search on all engines included in toolbar.
Swiftabs - You can move to the next/previous tab or close the current tab with user-defined keys.
Dictionary tooltip - Press ctrl+shift+D (or) double-click (or) right-click after selecting a word to see its meaning.
Open link in… - Adds context menu items for opening links and images in a new background tab (or foreground tab if your tabs normally open in the background), in a new background or foreground window, and in the current tab. Includes an option to gather menu items into a submenu.
Tooltip enhancer - See more on mouseovers
Autofill - Automaticall fill out forms
Searchwith - Search selected with various search services. It adds a “Search With” sub menu on the context menu with a configurable list of search services that can be used to search the highlighted text.
Tav Mix Plus - Enhances Firefox’s tab browsing capabilities. It includes such features as duplicating tabs, controlling tab focus, tab clicking options, undo closed tabs and windows, plus much more. It also includes a full-featured session manager with crash recovery that can save and restore combinations of opened tabs and windows.
Add bookmark here - Allows you to add bookmarks to any folder in the bookmarks menu like in Opera.
Always Remeber Password - Instructs web sites to always remember your password. Some sites like Yahoo Mail, Hotmail, and banking sites instruct the browser to never allow your password manager to retain your information.
undoclosetab - Reopen a closed tab.
PDF Download is something better than Acrobat Reader, but it did have bugs a couple months ago when I installed it. It is possible when setting the plugin options and Foxfire options, that the program can cause recursion. New tabs will keep opening and you have to use task control panel to End Task.
Session Saver apparently has a nasty memory leak - I’ve switched over to Session Manager. Clunkier, but I’ll live.
Mouse Gestures. You’ll wonder how you ever got around without it. You’ll wish they’ll have made something for navigating through general windows in Windows.
Spoofstick is a good one to install on a computer used by… shall we say… less experienced users - it tells you clearly what domain the web page you’re viewing belongs to - useful for identifying whether you’re logging into eBay or some scammer’s web page instead - experienced users aren’t at risk of doing something like that, but in some cases, it’s easier just to explain to auntie that if it says anything other than [whatever] in the big green text, then close the window.
I also went through a bit of a StumbleUpon phase - it was fun, but I haven’t used it in a while.
My single favorite is a simple one: Full Screen Homestar Runner. It makes homestarrunner.com pages appear fullscreen, maximizing the flash goodness.
I also really like Colorful Tabs, IE View (and Firefox View) and of course the Google Toolbar.