It depends on how many tracks you have going at once. You’ll notice that the place that probably needs rendering is the area of the timeline where there are more tracks than other places in the timeline. I’m doing a project right now that has lots of audio tracks and most of the timeline plays fine. When it gets to a place where there are 12 tracks instead of 8 I get the red line. I made a keyboard shortcut that renders audio only.
As far as firefox. I’d try turning off ALL the extensions in Firefox. I’ll bet it’s one of those that likes the ram that FCP is using and it has to page to work. Just a thought…..