See bug #1888802 for a very similar but slightly different description of the experienced phenomenon. Note that changing the window size (of the preview pane or the entire TB window) fits all the images.
Bug 1875325 Comment 8 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
See bug #1888802 for a very similar but slightly different description of the experienced phenomenon. Workaround: Changing the window size (of the preview pane or the entire TB window) fits all the images.