There were many changes between 16.04 that your friend is using and 18.04 that you're using, so it would be useful to compare with someone on 18.04, if you can.
The fact that this works with Chrome suggests something firefox-specific indeed. Can you try running firefox with a clean profile ("firefox -P") and see whether the issue persists?
Thanks for the feedback.
There were many changes between 16.04 that your friend is using and 18.04 that you're using, so it would be useful to compare with someone on 18.04, if you can.
The fact that this works with Chrome suggests something firefox-specific indeed. Can you try running firefox with a clean profile ("firefox -P") and see whether the issue persists?