One of the unique features of Firefox that is often underused is the ability to switch between style sheets. This functionality is accessible under ‘View > Page Style’ menu. Originally it was designed for swapping between themes provided by sites; however this never really took off and has largely been forgotten about.
This functionality can however be quite useful in certain circumstances. The ‘No Page Style’ option turns off all style sheets, making it easy to copy otherwise difficult to select content, or page elements that have been constructed to stop copying – like transparent images over other content. Which highlights why all such faux-protections achieve is hampering usability, if you don’t want something on the Internet copied, password protect the content or don’t put it on the Internet at all.
Even more useful for web development, is testing new site features with alternative style sheets. The alternative style sheet, as the name suggests, is added the html page in addition to the main one. Normal visitors to site won’t see any change, but the developer can select anyone of the alternatives and navigate the site with completely different styling.
This a handy way to test seasonal site theme changes, Christmas, Easter etc, in cases where there isn’t a staging server for the site in question.