Regarding the Firefox issue, I've made some small progress, but it's still not an optimal experience:
I couldn't get that "Import Settings" screen to go away, so I held down my remote Menu and - buttons until my AppleTV rebooted (conversely, holding down Menu and + will give you a Black Screen Recovery, let you reset the language and resolution, in case you got yourself screwed up there).
Under "Maintenance > Install Extras" I installed support for Remote HD.
I bought the Remote HD app ($4.99, and not sure it was worth it) for my iPhone from the App Store, and connected it to my AppleTV to have some kind of mouse cursor control. I found this doesn't work very well, but it seemed necessary to have some kind of mouse for Firefox. [I'm currently wondering if there's any way to get one of those USB/bluetooth mice (small usb bluetooth adapter) to work. That would be a much more reliable and responsive solution if it's possible... but I digress.]
I launched Firefox. This time it had a crash notification from the previous session not being shutdown properly. It asked to start a new session or try to recover the previous session. Using the Remote HD app on my iPhone, and with much frustration and some patience, I managed to get the mouse cursor to click on "Start a New Session".
The result was that Firefox actually launched with a browser window. The mouse cursor is horribly laggy for me on iPhone 4 using Remote HD, and the Firefox window is placed slightly off-screen, which is frustrating. The left edge of the window is cut off and there is a gap between the right edge of the window and the right side of my HDTV screen. This feels like wasted space. This is with the AppleTV set to 720p resolution. I tried running the app with my AppleTV set to 1080p and the same thing happened. In fact, the browser window in Firefox appeared the same size and resolution in both 720p and 1080p settings, making me wonder if that application actually downgrades the display resolution temporarily to run.
From my experience, sometimes launching the Firefox app takes seveal tries. Often nothing happens when I click it from Nito TV > Applications, and I have to wait a while, keep hitting 'menu' on my remote until it backs out to Nito TV and go back into Applications and try again. Eventually it launches.
I don't know if this will be of help to anyone, but I'll post it just in case.