"Unable to parse server JSON message" dirty fix

If you're having this error message while updating CSS or JS files, you can try this dirty fix:

Open file (userdata folder)\Mozilla\Firefox\Profiles\(your profile folder)\extensions\xrefresh@xrefresh.com\chrome\content\xrefresh.js

Find the line 134 (var buffer = UTF8.decode(parts[i]);)
AFTER, add:

var contentRegex =/"contents":\{(.*?):/i;
var regexpResult = buffer.match(contentRegex);
if ( regexpResult ) {
var path = regexpResult.pop();
var newPath = path.replace(/\\/gi, "\\\\");
var newBuffer = buffer.replace(path, newPath);
buffer = newBuffer;
}

Restart Firefox.

The problem is related to xRefresh monitor not escaping correctly path slashes.