firebug is the tool you need.
https://addons.mozilla.org/en-US/firefox/addon/1843
once you have it loaded -> then you can select 'HTML' in the left hand panel and in the right side panel under the 'style' tab you can change the css live. it's pretty cool.
here some other tools to pimp your firefox.
FirePHP
Extension of the Firebug add-on to show PHP debug information on Firebug console.
https://addons.mozilla.org/en-US/firefox/addon/6149
- YSlow
Currently another extension of the Firebug add-on to measure page loading times and identify problems that make your pages load slower.
https://addons.mozilla.org/en-US/firefox/addon/5369
- Web Developer
Several tools like disabling Java-script, CSS, redirection, clearing browser caches, resizing the browser window to test pages with different sizes, etc..
https://addons.mozilla.org/en-US/firefox/addon/60
- User Agent Switcher
Switch the browser identification to test how sites respond to different browsers.
https://addons.mozilla.org/en-US/firefox/addon/59
- HTML Validator
Validate page HTML to help finding page generation errors cause by eventual bugs in the PHP scripts.
https://addons.mozilla.org/en-US/firefox/addon/249
- Java-Script debugger
Run page Java-script code with single step support and ability to show Java-script variable values during debugging.
https://addons.mozilla.org/en-US/firefox/addon/216
- LiveHTTPHeaders
Show page request and response headers.
https://addons.mozilla.org/en-US/firefox/addon/3829