One of firefox killer-features is the variety of add-ons. Following is an overview of the add-ons I use currently.

Vimperator

Vimperator is a real gem! Adds some vim (editor) feeling to the browser. Makes you faster, because nearly all mouse action can be supplemented with keyboard shortcuts. Also automates more complicated flows with macros. At start using vimperator can be somewhat annoying because pressing some keys do unexpected things, but investing time to get used to it pays off definetely.

Xmarks

Xmarks saves your bookmarks to a server and makes synchronization possible between different machines. Very handy if you are working from different computers. Most likely it could be replaced by upcoming firefox 4 which offers this functionality in its core.

Web Developer

Web Developer is a nice webdeveloper testing kit. Numerous things can be done like style/CSS testing, gathering meta-information of the page, handling cookies, finding broken images.

Firebug

Firebug is a perfect accompany to Webdeveloper for testing/analyzing websites. Offers JavaScript debugging, analyzing DOM tree, viewing CSS styleor watching HTTP calls and request/response contents. It is also plugin aware (see below).

Firecookie

Firecookie is an addon for firebug. Makes cookies handling (reading, deleting, editing) much easier as with Webdeveloper plugin.

YSlow

YSlow is an addon for firebug, which offers performance test for webapplications. Gives a good overview how your site performs and gives a summary in grade style (A-F). If it gives you bad grade, still question whether they are appropriate in your special case (e.g. YSlow moans about missing CDNs, but an usage of a CDN doesn’t always makes sense or you don’t have any control over certain included components).

Live HTTP Headers

Firebug offers good HTTP traffic tracking. But sometimes I also use Live HTTP Headers because you can filter tracking HTTP calls by URL and content-type, for HTTP POST you can set your own defined payload.

JSONView

When testing webapps, instead of using curl sometimes it is handy to fire a HTTP request directly through firefox. If doing so by default firefox makes problems and prompts to save json (Content-type: application/json) as a file to instead of just displaying the content inside the browser window. JSONView bypasses this and displays json content appropriately.