The following is a list of the plugins I use with Firefox every single day. My hope is to demonstrate the power of the plugin system in Firefox and how you can customize Firefox for a more productive browsing experience.
The first plugin I ever installed and as such I found it proper to start out with this as it showed me the true potential of Firefox. Tabmix extends the existing tab options considerably by providing settings for almost every single scenario imaginable.
I use Tabmix to force links to open in new tabs instead of popping up new Firefox windows. The built in session manager (which the Firefox team implemented directly in Firefox) allows me to restore my previous browsing session automatically which is quite nice. Finally it allows me to change tab opening behavior so that new tabs pops up next to the active one and I've got it set up so that new tabs only get focus in specific scenarios which means that I'm able to go through a site and click interesting links without breaking my reading flow, when I'm done I can checkout the opened links at my leisure. There are tons of other possibilities with Tabmix Plus but those are the ones I use myself.
Adds a unique color to every tab open making it even easier to navigate your various tabs. An option allows you to have the colors generated based on the URL which means that you get the same color for the same web site each time. Additionally it just brightens up the interface considerably.
A good example of what's possible with Geasemonkey is the Gmail Super Clean script which allows you to turn the standard Gmail interface like below:
As you can see from the following screenshot I use Greasemonkey entirely for customizing Gmail but of course many scripts exist for other sites so be sure to check out Userscripts.org.
Coming from IE one of the this I missed the most was the ability to click Run on downloads. Firefox forced me to select a temporary location for every single download which in theory is well and good but in practice I sometimes just download stuff for one times purposes, e.g. I want to test something out, in those cases having to select a location is annoying as hell. OpenDownload takes care of this by adding a Run button to the standard Firefox download dialog. By clicking the button Firefox downloads to a temporary folder and launches the download when done.
It's one of those plugins that saved you a couple of seconds each time you want to just run something. In the long run it adds up, not to speak of getting rid of the annoyance itself.
Want to use that big fat pipe to the internets more efficiently? Fasterfox is a bunch of tweaks to Firefox which will allow you to do just that. Fasterfox works in two ways: Firstly It optimized various aspects of the network utilization such as multiple concurrent connections, caching, DNS cache, etc.. Secondly it prefetches links ala what Google has been doing for a while with Google Web Accelerator but with some limitations as to avoid "dangerous" behavior like logging you out from a site, emptying your shopping cart. The primary limitation is that Fasterfox will not prefetch dynamic pages such as ASPX, JSP which to my mind makes a lot of sense as I've never used prefetching due to the problems described earlier.
As you saw when I introduced Greasemonkey I use Gmail and quite extensively too. In fact I'm at a tipping point where using Outlook for my private mail doesn't make too much sense any more. In essence I'm using Gmail for everything but my calendar so that's the one reason why Outlook is still sticking around. But that's a different story :)
The function of Gmail Notifier as you probably deduced by now is to notify about new mail in your Gmail account. It adds a little icon to the lower status bar of Firefox which indicates the number of unread mails in your account and a notification is displayed on the desktop when new mail is found. Clicking the icon will bring you to your Gmail page via a secure HTTP Connection. Simple, effective.
The Google Toolbar, who can live without it? The toolbar is the first thing I install whenever I do a fresh Firefox install. Yes Google spies on my through this thing, yes my entire life is run by Google services but I don't give a damn it's so nice :) The latest version of Google Toolbar integrates even better with Firefox by adding an option to replace the standard Firefox search box next to the address bar. Makes for a more compact UI. My favorite feature of the toolbar is that search words are added as clickable buttons which finds the search term on the page. Supremely useful.
For all the usefulness of Firefox you simply can't get around the fact that Internet Explorer has had and still have a huge percentage of the install base covered which means that some sites simply aren't going to play nice with Firefox. IETab alleviates this problem by allowing you to switch rendering engine on a single tab enabling the IE engine to run within Firefox. IETab allows you to configure sites that should always be rendered using the IE engine, Windows Update comes to mind though only a problem in Windows XP as Vista does away with the Windows Update page in favor of a client app. Also available from the Firefox UI is a button for switching between the rendering engines on the fly.
Keeping bookmarks in sync between browsers were a priority to me when I was initially trying out Firefox as I was still very much using IE. I didn't want to commit to the custom bookmark system Firefox introduces so instead of keeping the two in sync I went and found PlainOldFavorites which provides access to the standard Windows/IE favorites from within Firefox. It adds a new menu item to Firefox called Favorites just like we're used to in IE.
With an increasing number of bookmarks I'm finding it harder to keep them relevant and up to date which is where del.icio.us comes into play. del.icio.us is a service for storing your bookmarks online much like what Google provides with their toolbar however del.icio.us was the original player in that particular space with more focus on the social aspects of bookmarking. For me personally it's just a matter of storing my bookmarks in a central place where I don't have to worry about backups. If you're interested in the social aspects of del.icio.us check out their about page.
Since I started using del.icio.us in Firefox I've noticed that I use bookmarks much more actively and I tend to tag (add) a lot more bookmarks to my collection. An unexpected side effect of del.icio.us is that I'm now able to see what similar pages other people tag within particular tags. Very useful that I can get additional bookmarks relevant to stuff I'm looking for which have already been prescreened by another person.
As a web developer I find myself in need of a FTP client from time to time. The fact that I don't need a client very often doesn't warrant a dedicated software for the FTP'ing file back and forth. FireFTP is a simple FTP client built right into Firefox which. An added benefit of having the FTP client inside Firefox is that it opens up in a separate tab which means you can have multiple connections open at the same time.
You cannot live without the Web Developer plugin if you're doing any kind of web development. As you can surmise from the name Web Developer helps you out when you're creating web pages. The focus of Web Developer is inspecting the HTML of your pages and understanding the structure of a page. It includes a number of validators for checking CSS, HTML, RSS, and a bunch of other stuff.
Firebug introduces a browser for inspecting the HTML structure in a hierarchical fashion which in conjunction with Web Developer makes for a very powerful development environment; all in the browser.
A neat little feature is an small indicator icon added to the status bar which shows the "compile" status of the web pages; essentially telling you whether any bugs exist in your page.
You can customize Firefox exactly to your liking as you can see from this list of the plugins I use everyday. The plugin system is easily the most powerful feature of Firefox; if you take the time to explore the potential. For me personally the plugins mean that I'm sticking with Firefox now that I've learned the potential. In many ways the plugins goes hand in hand with the reason why I'm using software like Resharper in my day to day development tasks: I want to get the most out of the time I spend behind the screen.
I hope that this list has inspired you to go look at the various plugins yourself and do leave a comment if you find useful plugins for yourself. You can find additional plugins at the Firefox Add-ons page; keep in mind that the Firefox team hasn't actually decided on what to call plugins so you'll see the terms plugin, add-on, and extension used interchangeably on both their web pages and in the actual browser. Honorable mention goes to Greasemonkey: the plugin system within the plugin system (it does get very meta at this point, right :)).