Below is a list of my MUST HAVE Firefox extensions. These are not all of the Firefox extensions that I have installed, this is a list of extensions I feel everyone should have. Let me know if there is a truly great extension that I don't have listed but should.
Mouse Gestures - This is a must have! Makes life so much easier. You will wonder how you lived without it after using it for a while. I can't believe how much quicker you can navigate the web with this installed.
FoxyTunes - Great music player addin. Sits in the status bar and works with pretty much every media player.
Firebug - Must have for web development. Easy to use and unobtrusive. Very powerful for exploring the DOM, CSS, and JavaScript.
IE Tab - This one allows you to flip to the IE rendering engine inside Firefox. Very useful for testing web apps. If you like me, you end up doing all your development work in Firefox and don't test in IE until the sites almost finished. This extensions makes it very quick and easy to get an IE preview in a new Firefox tab.
Download Statusbar - This plugin is a replacement for the default download manager. I like it because it doesn't open a new window when you start a download. It just shows that status in the footer. Plus there are some other cool options that you can set that make it better than the regular download manager such as automatically removing certain file types like PDFs. This should be worked into the default release of Firefox in my opinion.
FireFTP - Odds are you probably already have a favorite FTP program, but if yo don't, or would like one integrated into your browser this is a very good choice. I was a CuteFTP user for years and years, but I enjoy FireFTP so much it has become my default FTP client. Very clean and polished.
Resizeable Form Fields - I saw this extension on the Daily Grind and it has quickly become one of my favorite extensions. This nifty little add-on allows you to resize HTML text and drop down fields to make them easier to view or edit. This is great if you are filling out a form that only gives you a very small text area. There are some text boxes that it doesn't work on, but for the most part it is great to have around.
Modify Headers - A must have if you are a web developer. This allows you to change the headers sent out on each web request. I can't tell you how many times this comes in handy.
del.icio.us - I don't know if you use del.icio.us or not, but if you do this is great for bookmarking. It's nice to have those bookmarks available wherever you may be.
Inform Enter - Great if you have to enter the same information into fields over and over. I use this when submitting sites to directories. It saves a ton of time.
Dictionary Tooltip - If your like me, you browse the web a lot and come across quite a few words you don't really know the meaning to. My old method was to open a new tab, browse to dictionary.com and lookup the word. This nifty little extensions allows you to double click a word and instantly get the definition. Quite convenient.
ShowIP - I don't know if everyone will find this useful, but I like it. It shows in the status bar the IP address of the website your are currently viewing and allows you to do quick lookup information such as whois. One of the largest uses I find for this is I tend to test websites with their actual domains on my staging server by using static HOSTS entries. This help keep me aware of if I'm working on the live site or on the development server. It can be a live saver.
Clear Cache - This is an awesome extension that allows you to quickly clear the browsers cache by clicking a button in the toolbar. It's a great way to make sure you have the most recent images and CSS for a site.
Mouse Gestures - This is a must have! Makes life so much easier. You will wonder how you lived without it after using it for a while. I can't believe how much quicker you can navigate the web with this installed.
FoxyTunes - Great music player addin. Sits in the status bar and works with pretty much every media player.
Firebug - Must have for web development. Easy to use and unobtrusive. Very powerful for exploring the DOM, CSS, and JavaScript.
IE Tab - This one allows you to flip to the IE rendering engine inside Firefox. Very useful for testing web apps. If you like me, you end up doing all your development work in Firefox and don't test in IE until the sites almost finished. This extensions makes it very quick and easy to get an IE preview in a new Firefox tab.
Download Statusbar - This plugin is a replacement for the default download manager. I like it because it doesn't open a new window when you start a download. It just shows that status in the footer. Plus there are some other cool options that you can set that make it better than the regular download manager such as automatically removing certain file types like PDFs. This should be worked into the default release of Firefox in my opinion.
FireFTP - Odds are you probably already have a favorite FTP program, but if yo don't, or would like one integrated into your browser this is a very good choice. I was a CuteFTP user for years and years, but I enjoy FireFTP so much it has become my default FTP client. Very clean and polished.
Resizeable Form Fields - I saw this extension on the Daily Grind and it has quickly become one of my favorite extensions. This nifty little add-on allows you to resize HTML text and drop down fields to make them easier to view or edit. This is great if you are filling out a form that only gives you a very small text area. There are some text boxes that it doesn't work on, but for the most part it is great to have around.
Modify Headers - A must have if you are a web developer. This allows you to change the headers sent out on each web request. I can't tell you how many times this comes in handy.
del.icio.us - I don't know if you use del.icio.us or not, but if you do this is great for bookmarking. It's nice to have those bookmarks available wherever you may be.
Inform Enter - Great if you have to enter the same information into fields over and over. I use this when submitting sites to directories. It saves a ton of time.
Dictionary Tooltip - If your like me, you browse the web a lot and come across quite a few words you don't really know the meaning to. My old method was to open a new tab, browse to dictionary.com and lookup the word. This nifty little extensions allows you to double click a word and instantly get the definition. Quite convenient.
ShowIP - I don't know if everyone will find this useful, but I like it. It shows in the status bar the IP address of the website your are currently viewing and allows you to do quick lookup information such as whois. One of the largest uses I find for this is I tend to test websites with their actual domains on my staging server by using static HOSTS entries. This help keep me aware of if I'm working on the live site or on the development server. It can be a live saver.
Clear Cache - This is an awesome extension that allows you to quickly clear the browsers cache by clicking a button in the toolbar. It's a great way to make sure you have the most recent images and CSS for a site.