What development plugins do you use for Firefox that you could not live without?

Just a couple weeks back one of my co-workers was using the Web Developer plugin for Firefox which I had never seen before, and I was wondering if there are any other great development plugins for Firefox that you cannot live without as a web developer.

The ones I currently use are:
Web Developer
Firebug
ColorZilla

Edit: Please give a short description as well

Duplicate of Essential Firefox Plugins/Extensions? (related to Web development of course)

From stackoverflow

• livehttpheaders - Easy tool for capturing browser's http conversation with server.

  jmein : +1 I think I am gonna have to try that tool out

  Luc Touraille : Could you please provide a quick description of the plugin? I know Google is my friend, but we should be able to have an overview of the different plugins just by reading the answers, whitout having to go back and forth to other websites.

• Only 2 I could not live without:

  Firebug

  YSlow

  jmein : +1 YSlow sounds like another good one

  ayaz : Thank you for mentioning YSlow.

• Dust-Me Selectors finds unused CSS selectors in your site. You can use it to trim away the excess from your CSS files. It occasionally also supports FireBug in the role of making sure the CSS rules you are trying to apply are actually applying.

  Well, I guess I could live without it, I'd just have CSS bloat.

  jmein : +1 that sounds like a really good tool

  Anton : Very cool, I will have to try this out!

  ayaz : Interesting. Thanks for mentioning Dust-Me Selectors.

• Firebug is my favorite.

• Anybody not using Firebug should be.

• • Firebug
  • Web Developer
  • DOM Inspector

• • Live HTTP Headers: See any HTTP POST/GET request and it even allows you to 'replay' these requests (after editing it to your liking).

  • Add & Edit cookies: Add and edit cookies :)

  • Firebug: For obvious reasons.

• There's quite a few I really like.

  View in IE This is great for checking for cross-browser compatibility

  Firebug This has been mentioned before many times, but its usefulness is just amazing. From javascript debugging to live-editing of anything, its a very powerful tool to have.

  YSlow This one is actually pretty cool. It analyzes your site and give you it a performance grading along with suggestions to make your site faster.

  Fireftp Not a "developement" tool per-say, but its useful for me at least, to send files back and forth to my webserver.

• In addition of the previously listed plugin, I use MeasureIt, which is quite usefull to measure parts of the web pages (interesting for design, CSS...)

• The ScreenGrab plugin is worth a mention - saves going back and forth between the browser and a graphics editor.

  It can also take screen grabs of the whole page which is quite useful.

  Obviously not a day-to-day usage kind of thing but worth having on occasion.

• Aside from those already listed here, I use UserAgent switcher and wml browser for mobile development

• foxmarks - A perfect bookmarks synchronization plug-in that is very well updated and I never lost one bookmark while using it on +/- 5 different computers.

• Greasemonkey, Firebug, ColorZilla, Flashblock, Live HTTP Headers

  jmein : +1 ah forgot to mention greasemonkey. I got that too

• • Auto Copy (http://autocopy.mozdev.org): automatic copy of selected text

  • Firebug (http://www.getfirebug.com/): the debugging tool for e.g. javascript. Also you can modify CSS on the fly to see what has changed (nifty feature!)

  • HttpFox (http://code.google.com/p/httpfox/): monitors and analyzes all incoming and outgoing HTTP traffic between browser and webserver

  • IE Tab (https://addons.mozilla.org/firefox/addon/1419): quickly switch the render engine to IE of the current tab.

  • Quick Locale Switcher (http://www.captaincaveman.nl): switch locales to check the multi language functionality of your website

  • Screen grab! (http://andy.5263.org/screengrab/): create screenshots of a selection, the visible portion, the whole page and save to clipboard or file.

  • Web Developer (http://chrispederick.com/work/web-developer/): THE web developer toolbar

• • Firebug
  • All-in-one gestures. I've actually recently got unused to them since I switched to Chrome, but while I was using firefox they were indispensable.
• • Firebug (you MUST get this one. The logging console alone is worth its weight in gold. No more need for those annoying alert() calls in Javascript.)
  • FirePHP (debugging tool for PHP scripts to send data to Firebug console)
  • FireCookie (manage cookies)
  • LiveHTTPHeaders (helps examine HTTP headers)
  • CacheViewer (helps examine files in the cache)

• I can't believe nobody mentionned Pixel Perfect!!

  Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.

  By toggling the composition on and off, the developer can visually see how many pixels they are off in development. Pixel Perfect also has an opacity option so that you can view the HTML below the composition. By being able to see both the composition and the HTML you can now simultaneously use Firebug while Pixel Perfect is still in action.

• In addition to already mentioned plugins (Firebug, Greasemonkey, Web Developer, occasionally a few of the others), i can recommend these:

  1. Stylish - Customize the look of the application and of websites with Stylish, a user styles manager.
  2. Ubiquity - An extension that allows for the use of dynamic commands in Firefox.
  3. SQLite Manager - Manage any SQLite database on your computer.

  Of course, many plugins are not directly development related, but very helpful nonetheless, like Scrapbook (helps you to save Web pages and organize the collection), Adblock/Adblock Plus, CustomizeGoogle, PlainOldFavorites (use IE favorites directly from Firefox), Tab Mix Plus, and more.

  EDIT: corrected link for Scrapbook