How funny; I opened IMHO to search on just this topic, and here’s this thread!
So I just installed FF… any advice for a newbie? What extensions are a must for new users?
All input appreciated! 
Here’s my list:
Adblock
Allow Right Click
Image Zoom
Tabbrowser Preferences
BugMeNot
MapIt (lets you highlight an address, right-click and find a map)
Plain Text Links
In my searchbox I have Google, Google Images, IMDB, TV Tome, Wikipedia, A9 and Babelfish English -> Spanish. Searchbox addons are at mycroft.mozdev.org.
Jump Link is very useful - helps get around those annoying sites (such as Hotmail) that force every link to open in a new window within frames.
Tabbar Extensions might not be the best, but then again they might be the single most useful extension for you. It depends on how tolerant of complexity you are when it comes to altering how your browser behaves.
One thing I can think of that a newbie might like about Tabbar Extensions is that it comes with a little toolbar button that remembers the last ten (or so, the number is configurable) tabs you closed, so you can reopen them at will. This is great for that ‘Oh, shit’ moment when you realize you closed a tab you didn’t mean to.
The Googlebar is one of those extensions you come to rely upon and don’t notice until it’s gone. You really should download it and you will get addicted once you have. If you already use it on MSIE, you know what I’m talking about.
Other than that, get to know how to get around Mozdev. Do some Googling of your own. There’s a great big world of extensions out there.
GuanoLad: In the Dune series of novels, it’s the first words of the Bene Gesserit ‘Litany Against Fear’:
I suppose you need to have read the books to get the full effect.
Does anyone know of an extension that provides a roll-over preview of the name and file format of an image on a web page?
Hmmm. This is one I wanted, but have never gotten it to do anything. I’ll have to try installing it again and see if maybe it works now.
One of my recent additions that I really like is the Clusty Toolbar. I don’t use it for it’s “main” feature, rather I use one of it’s sub-features. I minimize the toolbar it creates. However, you can highlight words or phrases and and right click, and it will create a floating window with either the Wikipedia entry or Dictionary entry for your selection, based on what you right click. Very handy.
Nuke Anything. For the times when you don’t want to permanently black something (ala AdBlock, as mentioned above), but just want to block it for right now. I have found it useful on sites where navigation menus (such as the news menu on msnbc) blocks content when you move your mouse around.
Ooooh. I’d just reinstalled Firefox at home after screwing up an older version of Mozilla (which was the Straw That Broke The Camel’s Back).
The other descriptions of Tabbar Extensions were tasty, but this is the one that closes the deal for me. Shall be grabbing it once I’m home.
glee
(I usually have several windows with lots and lots of tabs open that I leave up for days, which leads to Horrible Incidents where I close the wrong one by accident)
I just downloaded Firefox (love it), and found this thread very useful. What I’m wondering is how you can set Firefox to use a quick launch. I’ve been using Netscape 7 for a while, and really like how quickly it loads (because it’s got a quick launch icon in the taskbar). Is there a way to do that for Firefox?
Most of the extensions that I use have been mentioned (Tabbrowser Extensions, Foxytunes, ForcastFox), but there is one that I couldn’t live without.
SpellBound. A simple spell check program that can be used to check online forms or other text entries. Basically, anything you type into Firefox, it will spell check for you. Very handy for a guy like myself whose spelling abilities are somewhat subpar, at least when typing. Just right click in the text box, and it will be on the pop up menu. No more writing posts in word and then C&P’ing them! It also has several language dictionaries, and it is quite good at picking up missed words. The only thing that it lacks is a grammar checker.
I second Adblock, and here is an excellent adblock filter. It seems to block just about everything, including those Geocities boxes.
Pipelining is not really an extension, more of a tweak to make pages load up faster. Do the “Speed up page rendering” and “Enable Pipelining” in about:config. YMMV on this one, but darned if it doesn’t make my surfing go a little faster.
Allow Right Click gets around those sites that disable your right-click to cut & save pictures, text, etc.
Sometimes web sites resize my window to full screen. Are there any extensions that prevent this? That leave my firefox window the same size?
Like Derleth, one of my favourites was Tabbar Extensions; then I was doing a clean install of Firefox and downloading my extensions again, and I happened upon this:
and the Newspeakish pronouncement
I now have Tabbrowser Preferences. Not as many features (I especially miss being able to drag-and-drop tabs), but better than nothing.