I think I just figured out why this was happening to me on both firefox and chrome (OSX Sierra). There are firewall rules that will sometimes block your connection to the dtube servers.

For example, if you see something like:
"Deny outgoing connections to dtube2.gateway.ipfsstore.it"
"Deny outgoing connections to dtube1.gateway.ipfsstore.it"

That blocks any of the uploads from working. In my case, I am using little snitch, so I was able to adjust these rules.

Just uploaded my first video w/ chrome, and tested a snap/thumbnail failed upload on Firefox after disabling the rules, and that now works.

Give it a try and lmk if that works.