If this is a fresh install, this probably doesn’t apply, but may if it was previously installed and you are only now using it seriously. Just as a general rule for browsers and computing, clearing cache or temp files often resolves issues with hanging. Sometimes stale data leaves the system to time-out waiting for information that can never arrive.
In Firefox, it’s the “Cache” check box in Settings | History | Clear Recent History… Be sure that “Everything” is the time range in the box at the top and that no other boxes are checked unless you want them to be.
Sure, I’d be happy to send some your way. They are the same size I’ve always taken and they work fine in Chrome, though, so I don’t think that’s the issue.
Just an update: recently I’ve uninstalled and cleared Firefox and then reinstalled it, and after trying to upload, it still doesn’t work… I’m not quite sure what the issue is, though.