Aesthetics
Yes, Firefox allows you to easily theme (or skin, as the kids say) your Firefox browser. Some classics? The Woody Firefox for surfer dudes, the Whitehart Firefox for the minimalists, and Littefox for those wanting to see more pixels of the content at the same time. Lastly, if you want your brow-brow to have more bling-bling, checknizz the PimpZilla themenizzle.
These are just a few I found interesting. The list goes on and on. Point out your favorites below.
Performance
Fast is good. Fast and stable is even better. If there's one thing users of Opera and IE can usually win in an argument, it's the idea that Firefox renders ... s-l-o-w. If it was just IE, you could pass it off as IE is built into the underlaying OS for the most part. Opera, though, is an independent and has zippy fast rendering capabilities. In any case, there are Firefox configuration files you can tweak for a smoother browsing experience.
HTTP pipelining
Some swear this is a great tip for speeding up Firefox. However, it might be like putting nitrous in a car's fuel system. That is, it could be great for a while, but might harm the way the car works if you're not careful. Adot's notblog at MozillaZine has a disclaimer for enabling HTTP pipelining
From one source of the tip (Check out all the good comments on the age too):
1.Type "about:config" into the address bar and hit return. Scroll down and look for the following entries:
network.http.pipelining network.http.proxy.pipelining network.http.pipelining.maxrequests
Normally the browser will make one request to a web page at a time. When you enable pipelining it will make several at once, which really speeds up page loading.
2. Alter the entries as follows:
Set "network.http.pipelining" to "true"
Set "network.http.proxy.pipelining" to "true"
Set "network.http.pipelining.maxrequests" to 8. This means it will make 8 requests at once.*
3. Lastly right-click anywhere and select New-> Integer. Name it "nglayout.initialpaint.delay" and set its value to "0". This value is the amount of time the browser waits before it acts on information it receives.
If you're using a broadband connection you'll load pages MUCH faster now!
* - This originally said set it to 30, but someone who knows said that setting it above 8 wouldn't do anything.
The thing to consider, though, is how many simultaneous requests at a time are considered 'polite' before Apache denies you access with mod_dosevasive or some other method. Remember, you don't want to tick off a webmaster.
Recompiling
It should be possible to get speed increases by compiling your own OS native version of Firefox. MOOX, for example, has some great performance tuned builds available for Windows.
Tabbed Browsing
Tabs are a wonderful tool for browsing the web. I honestly don't know how I ever surfed without them. (I said the same thing after leaving text-only Lynx days. (The latter I still use, though.) There are quite a few extensions out there that mess with tabs, trying to expand their functionality. Tab Browser Extension (also known as TBE) and Tabbrowser preferences are two that look promising at the moment, but still need some work before I install them fulltime.
There are some things you can do to make tabbed browsing better, though, like new windows opening in tabs instead. Also nifty, is getting rid of the close tab button on the tab bar (while leaving it on the side bar.) If you've accidentally closed a window one time too many, try typing the following in your userChrome.css file:
/* Remove tabs close button */
#content .tabs-closebutton {display: none ! important;}
Cool Extensions
The official Firefox extensions website has more extensions than you can shake a stick at. (Please don't ask how many that it. I'm not sure either.) One of the first extensions you should install is xMirror. Why? Well, the official extensions site isn't the only one out there. At least three others have a lot of great extensions not available on the official site for one reason or another. xMirror allows you to see those extensions in the Firefox Extension manager
SessionSaver - Do you find yourself opening the same multiple tabs each time you get on the web? If so, this extension will be useful. It allows you to 'preload' a bunch of websites with one-click and in separate tabs. It even saves where the cursor was on the tab you were browsing.
miniT(drag+indicator) - Click, drag, and rearrange tab order. 'Nuff said.
IE View - I can already hear you asking, but without tabs, why would anyone want to open IE!? Well, unfortunately, some lazy programmers out there still code IE only versions of websites. Also, if you're a web designer, you probably check how a page renders in different browsers a lot. In any case, this extension adds an 'Open in IE' option when right-clicking a link.
User Agent Switcher - You will get this for one of two reasons. One, your friend has Opera and is always talking about how Opera allows you to send out a phony UserAgent while your piddly Firefox doesn't. The second reason you'll want this is if you're into Search Engine Optimization and/or like to see cloaked pages in action by pretending to be a search engine spider.
Word Count - Primarily useful to writers, this extension allows you to highlight text in the browser, right-click and get the total number of words in the selected area. Nifty.
Web Developer - This extension gives you a toolbar that makes a lot of nifty web designer tools available. From validation, to a header checker, to an outliner, the web developer extension makes a web designer's life a lot easier.
Linkification - This user installed extension changes plain text URLs on webpages into clickable links. It's a shame no one has thought up a tool like this. I mean, the coolness of installing something that creates links where there were no links before... Wait, where's the outrage that Google is facing for a similar tool? Hmm...
ColorZilla - This extension gives you a color picker that can zoom in and grab any color to the clip-board so you can cut and paste into other programs.
FoxyTunes - Tired of taking the three seconds it takes to bring your favorite media player (Winamp in my case) to the front to change your music or stop it? Well, now you don't have to waste those precious few seconds. This extension embeds the basic controls of your media player into the browser. They also have a nify playlist.
Define Word - With this extension, you can highlight text, right-click and search for the definition on Google.
Marijuana Delicious Delicacies - Quit using normal cookies and replace them with marijuana cookies and help make the world a more relaxed (if not forgetful) place.
SwitchProxy Tool - Not that you'd ever want (or need) to use proxy servers for browsing the web, but this extension allows you to quickly and easily switch between proxies while using Firefox.
Translate - Easily translate highlighted text with Altavista's Babelfish or Google's translation service.
ForecastFox - This extension will give you a quick glance at the current weather so you know whether you need your galoshes and/or umbrella.
Mouse Gestures - If you're into that kinda thing (who am I to judge?) you can check out a wide array (well, ok, seven currently) mouse gesture extensions that let you invoke commands just by drawing gestures with your mouse.
FlashBlock - This blocks Flash animations from running on a page. You do get a placeholder, though, with a button to play the flash if you desire.
FireFTP - This will let you open a simple FTP client in a new tab so you don't have to open another separate program. (Currently still in beta, but it runs well. More features are on the way...)
Gcache - This adds another feature to your right-click menu - see the Google cache of the current page. Handy if a site disappears or goes down.
WebmailCompose - This opens up a webmail app instead of Outlook or another external email program when you click on mailto: links on a page. (Currently supports GMail, Yahoo, Hotmail, Netscape, OperaMail, Mail.com, Horde, Squirrel, and one user-customizable entry.)
URIid - This allows you to apply user stylesheet (CSS) rules on a site-by-site basis. Why would you want to do it? Well, things like a customized Gmail interface.
BugMeNot - If you surf a lot of news websites, this handy extension allows you to easily fill in the first username and password associated with a site in the BugMeNot database. If you installed this when it first came out, give it another look because they've made it even easier to bypass forced user registration.
Hit-a-Hint - This allows you to use the keyboard to 'click' links. It takes some of the features we all loved with Lynx and makes them available with a GUI browser.
There a lot of other extensions out there. These are the ones I've found useful and/or interesting. Please leave any I've missed below. Also note that you can grab an RSS feed of the latest Firefox extensions.
Going Viral
Want to help Spread the Fox? If so, you'll probably want to get one of the many Firefox banners, buttons or flashing widgets. Display them on your website and help Spread Firefox. Going viral never sounded so dirty.