Hi I am a Ubuntu user myself and have experienced the same issue with audio over the web. One thing I do when that happens is close a few tabs and reduce down the amount of web content I am viewing. Restarting the media then seems to clear up the "static" type noise and everything is fine. I do not think that this is a Firefox issue and you may want to consult one of the (really helpful) Ubuntu support channels. You can find details of these [https://www.ubuntu.com/support/community-support here]. I hope this helps.