Firefox really makes you more productive when developing web applications or websites. Not because of its good browser engine - but because of the countless add-ons. I would like to present some of my favorite plugins which are a must have for sophisticated web development.
console.log("hello world") you’ll get a log message. It also measures network activity. And much more.
The Web Developer Toolbar allows you to disable or enable your cache. It can display your layout in several resolutions and makes the management of cookies and other relevant browser settings really easy.
YSlow measures the network performance of your website. Especially when developing high traffic websites, where every single byte counts, YSlow provides a lot of hints on how to improve the loading times of your pages.
If you’re on Windows and you need to do testing for our beloved mainstream browser, IE Tab might be the right plugin for you. Never ever open the Internet Explorer again, just load a tab with the IE ActiveX control. Saves a lot of time.
You probably want to search the plugin directory yourself - there are a lot of neat extensions. Be careful, though - the more plugins you load, the more memory your browser will consume and the slower it’ll get. So only enable a plugin if you really need it.
In case I forgot a good tool just drop a comment.