I see the problem with the example page you gave. Regrettably, I am unable or unwilling to make the extension work on that page or ones like it.
Normal web forms have a limited number of types of "elements" or "inputs" represent things like boxes for typing text, check boxes, selection menus, etc. (These elements have names like "input", "textarea", "select", and so forth.) The types of elements that can be in a form is standardized.
My extension works by detecting changes in the content or state of these standard form elements. When the content or state of one of these elements changes, the extension saves the new content or state on your computer, so that if you come back to the same form, it can refill it with the same values.
The page that you linked to, which is for composing a reply to a discussion, does not use one of the standard form elements to provide the box where you type your message. Instead, it uses a piece of software called TinyMCE, which provides a rich editor supporting things like font styles and bulleted lists. The box that you type into is not a standard form control, and furthermore it is inside an iframe, so technically it's not even on the same web page.
There is, in fact, a textarea element that the TinyMCE editor will put the contents of your message into when you click Reply. Unfortunately, this textarea does not get updated when you click outside it (or "blur" it), as normal textareas do. Thus, my extension is not able to save its contents.
The reason I wrote "unable or unwilling" is because, yes, with sufficient effort, it would be possible to support pages like this. But it's more effort than what I'm willing to put in. The problem is that there are too many variables; it would be virtually impossible to make the extension work as expected in all cases.
In fact, there are now many such non-standard forms on the web. The impossibility of supporting all of them (with reasonable effort) is the reason I no longer have a link to Form Autosave on my web site.
I'm sorry.
Chul