Quote Originally Posted by mr.sumo.snr
Could some explain to me "like I'm a five year-old" how I get a new window to pop up in Firefox which is an exact copy of the page I'm currently browsing?
Exact copy? Don't know, but perhaps there is some extension or plug-in which does that. Just browse the FF-website for that.

I use a different solution: right-click on "Quote", choose "Open in new tab"
Then you can write your response in the new tab & browse the thread in the other. I always do that.

If you're already in the "Reply"-window, you could just the same right-click on the thread title to open it in a new tab.