StyleSelector 
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StyleSelector was an extension to add stylesheet switching to Mozilla Firebird (now called Firefox).
That feature is now built in, though it lacks the persisting of the choice of stylesheet from one page view to the next that StyleSelector had (implemented using cookies).
There was also an extension for StyleSelector called UserStyles, which added the ability to apply your own stylesheets to pages, though it was very much in alpha stages. Maybe I'll revive it...