This extension does not honor the "thumbtime=" attribute when showing (stills from) video files.
The gallery page
has several video files embedded along with a "thumbtime=" attribute that controls which frame is used as the still for the thumbnail. The first file (Artefact comparison ToS f11811.webm) on the gallery page is represented by a thumbnail of frame 0 (reading "VP9").
When starting the "gallery slideshow", the file is represented inside the MultimediaViewer view by a thumbnail of frame 3 (reading "AVC").
Behaviour of MultimediaViewer would be much improved if it also showed frame 0, instead (– or if it played the fucking video!.. Why is this extension called MultimediaViewer although it does only really handle still images?..).