When developing new websites, one of the worse things is that you always have to refresh the whole page to see some minimal changes made on an external .css file. Sometime ago I have found one small peace of script that forces the website to just update the CSS code.
To create this button you should click with right mouse button on your “bookmark toolbar” in Firefox. In the menu you should choose “New BookMark”; insert some name, like “CSS Refresh” and in “location” paste this code:
Now, everytime you’re in Firefox making some changes on your CSS code, just press the button to update your style sheet.