I’m picky when it comes to Firefox extensions. I only have a couple of them installed. One of them is Check all by Emil Hesslow. I’ll quote him here:
This is a very simple extension that only do one thing but it do it very well. The way this extension works is like this: You select one or several checkboxes and right click. You will then get the option to “Check all selected checkboxes” or “Uncheck all selected checkboxes”. Perfect for sites that don’t have a “check all” option.
Great job, Emil! Especially if you use webmail or if you moderate a lot of forums.