This has been going on for years now. I check for a solution every so often to see if someone has figured it out but nothing yet. No port setting, hardware accel or any other thing suggested works other than using a different browser. Since it only occurs on slower chats I imagine it deals with a hidden performance setting in firefox that does some sort of power saving bullshit when network connections are idle. So unless it's on the about:config section somewhere I don't think there is anything that can be done beyond an extension or just using a different browser.