File upload works in Chrome but not in Firefox

[woepwoep]

when uploading one or more images using any of the four methods,
it works fine when using Chrome, but with Firefox after selecting the images from the client and pressing 'Open' button to return to the YAG backend, i do not see the upload progress bar like i do in Chrome.

It looks like a jQuery kind of problem, and there is no button to manually start the upload.

What could this be?
TYPO3 v7.6.18
YAG v4.2.1

Thx Ron