The Firefly image toolbar has a "fit image to frame" button (with four arrows in a magnifier) and a "fit image width" button (with just a left-right double-ended arrow). The mouseover help for these buttons is not helpful in distinguishing them:

fit-to-frame: "Zoom the image to fit into the visible space"
fit-width: "Zoom the image to Fill the visible space" (capitalization: sic)

The first should say "Zoom the image to fit entirely within its frame" and the second "Zoom the image to fill its frame horizontally".