Palemoon and Basilisk browsers: mp4 expansion not working

[43tbo8ca4]

.mp4 expansion not working for Palemoon or Basilisk. 4chanX 1.14.23.2 with Greasemonkey.

Incidentally I tried using the same set up in FloorP browser and it works perfectly.

I read the recent #3523 but even though the symptoms are the same, the cause is not related.

Thanks!