Like you, I paid for the HD membership assuming that I'd get 1080p video in Firefox but I subsequently discovered that Firefox is only given a maximum of 720p. I see in this thread there's a Firefox version of the extension so I'll try that.
If I watch Netflix in Safari I get 1080p video and I've been using Safari exclusively for Netflix because of this. Here's the Netflix system requirements page where they outline the maximum resolution for each browser:
https://help.netflix.com/en/node/23742
You can also see the details of your currently playing video by pressing Ctrl+Shift+Alt+D (and press again to dismiss it). It'd be nicer of Netflix if they made this information accessible via a menu item rather than an obscure keyboard shortcut.
All the more reasons to watch Planet Earth on blu-ray.
So worth the hardware.
For what its worth, a lot of 3rd party content is actually restricted to 480p (at least in Linux/Chrome). That, is definitely noticeable.
And then there are the Netflix Original shows that don't even get physical release.
The only people that "notice" are those with a private movie theater room.
Could be similar for HD content as well - if you have a poor internet connection, you might not even be getting 1080p.