One can use the nsIStyleSheetService in Firefox to load and register stylesheets which apply in a similar way to userstyle.css; they are applied to every page loaded. However, this service takes sheetURI, which is a URI pointing to a static file to be included as a stylesheet.
Is there a way I could actually generate this stylesheet in Javascript instead, on-the-fly? Or would there be another interface which allowed me to register a JS callback to output the desired stylesheet to apply, instead of a sheet URI?
Generate the CSS in JS, then base64 encode it and prepend data:text/css;base64,
Then, use this as the URI.
For example: body{background:red;}
Becomes: data:text/css;base64,Ym9keXtiYWNrZ3JvdW5kOnJlZDt9