Fireworks always opens
.jpg files and all other bitmap formats, with a canvas that fits exactly to image dimensions. The canvas can only be different for editable vector formats like it's native
.png for earlier versions) and some others like Photohop .psd.
If you want canvas dimensions to match all content (bitmap/vector or anything) there is a 1-click button: Deselect anything (for ex. click outside canvas), and then click
Fit Canvas in the
This works also perfectly if you are trying to import
.jpg files on existing canvases by dragging the file in it, pasting it, etc.
When you are creating a new image, I suggest to copy the bitmap to the clipboard. If you do this, when you create a new canvas
File > New, the dimension fields will already be filled for you.