$x(xpath) to handle $x(xpath, contextNode) too (catering relative xpaths) and to return strings, numbers or booleans, when the expression resulted in such output.$X is a variant on $x, which will return a node rather than an array, when the result was a node set. Instead, you get the first match, in document order. You'd be surprised how comfy and useful that is. These tools cut down the user script development feedback loop overhead for me quite noticeably. They also work even in a framed environment (when you pass a context node), which the former Firebug $x did not.After patching up the build script a bit until it worked (so I could test it out), I submitted the patches to the Firebug list, so they might end up in the upstream 1.0.6 build as well.
Relative XPaths are very useful for answering lots of intricate questions about web pages that you'd have to work for quite a while with the DOM inspector or Firebug's HTML view to figure out. If you like me like to bookmark comments you've written on some web page, you ideally want to find the closest anchor before the place where your comment showed up. Good blogs have easily clickable links around the comment to help you do that; others, like Ajaxian, don't.
With this hack, you find the closest preceding anchor by invoking Firebug's Inspector, picking out your comment, opening the Firebug command line (Command or Control + Shift + L), and typing, for instance,
node = $X('preceding::*[@name or @id][1]', $0); location.hash = '#' + (node.id||node.name);Then it's just the matter of bookmarking as usual. Since Firebug's console features are not exposed to the page, you unfortunately can not make a bookmark out of it, but it's nevertheless a good example of what you can do with this.