"Rocker gestures" sometimes don't change the mouse cursor appearance when they should

[UserNaem]

Firefox 54.0a2 (2017-04-18) (64-bit)
Steps to reproduce:

1. Enable rocker gestures in FireGestures's settings, leaving the assigned actions as default ("Previous tab" and "Next tab")
2. Open any image in new tab, e.g. https://qt.vidyagam.es/EB9b3sw.jpg (SFW)
3. Hover over the image (the mouse cursor should change to "zoom in" or "zoom out" loupe) and perform the rocker gesture
4. The mouse cursor will continue to look like the "zoom in" or "zoom out" loupe until you mouse over any part of Firefox's user interface, or return a tab with any image and un-hover the image.

Expected result is that the cursor should revert to normal.

Note: I was unable to consistently reproduce this behavior, sometimes the cursor is reverted to normal, sometimes it's not. Maybe it depends on what type of content will be in the tab you're switching to from the image tab.