Per-site user stylesheets
I suggest making a couple of minor modifications to Ryan’s code. Firstly, it over-writes any existing body tag IDs which could cause problems on the few sites that use a body ID as a hook for per-page styling. I would recommend not over-writing the ID if it already exists, or adding an additional class to the body element instead. More importantly, Ryan’s technique couples your browser’s user stylesheet to an XBL file hosted on his server. Not only is this a performance menace (for both his server and potentially your browser) but it opens a cross-site scripting vulnerability in that you are executing code from outside your control every time you visit a web page. I recommend hosting the XBL file locally instead, although the best solution would be to upgrade Ryan’s hack to a fully self-contained FireFox extension.
Quibbles aside, it’s a brilliant workaround. Be sure to check out the comment from michael on photomatt.net as well.