How to change a firefox option on a quick way? Shorcuts. Command-line

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http://unix.stackexchange.com – In Firefox we have two options at Firefox->Preferences->Preferences->Fonts and colors->Colors menu, Use system colors and Sites can use other colors. I would like keep the first one checked (and this is ok) and change the second on a quick way. A quick way could be pressing a shorcut on keybord, running a terminal command or changing a content of a config file (because I can do a shell script and use a keyboard command). My motivation is because I would like use my system colors always, but, if a webpage have a strange visual, I'd like change it to the original quickly. Any ide (HowTos)