I'm not sure how that programs works but... If the mask is a vector object you can select it using the subselection tool and then edit/move the individual points. It's how I tighten up a selection made using the Pen tool.
Just to add.... The pen tool also has a Redraw Vector Path tool under the little triangle. It's kind of tricky to use, but it does work. It helps to zoom in. Select the object you want to modify, then select the Redraw Vector Path tool. Move the tool over the path to modify. When the cursor changes the path to red, you can redraw the path. I'm not sure, but it seems to help if you start your redrawing on a node.
If you're not familiar with Masks in Fireworks, they can be vector objects or bitmap objects. Where the object is white, the underlying masked object will be fully opaque (visible). Where the mask is black or transparent, the masked object will be transparent. Where the mask is grey or semi-transparent, the masked object will be proportionally semi-transparent. To apply a mask, select both objects, the Modify > Mask > Group as Mask.
I am not sure how far back Auto Shape Tools go, but they can be used as Masks
Place the Auto Shape above the Layer you plan to Group As Mask.
Select both of them, then Modify > Mask > Group As Mask.
This allows you to use the individual 'achor' points on the mask to redefine it.