The size, visibility, and function of the lower-right-corner "window stretch(resize)" icon in the Firefly table options dialog vary from browser to browser.
On Safari, the icon is not visible at all, and the stretch function is not available even when the mouse is positioned in that corner.
On Chrome and Firefox, the icon works but its visibility varies greatly from platform to platform. It may be a fixed number of screen pixels in size? In this case, it become almost invisibly small on high-resolution screens.