"I've been using Perfect Viewer for .cbz/cbr"
Hi I use both Perfect viewer and comic reader mobi for my ever growing CBR/CBZ collection.
I use perfect viewer in landscape mode on my first gen kindle fire as its auto navigation is perfect in that orientation.
I use Comic render mobi for those titles where the author failed to be conservative with the amount of text per panel.
this is because Comic reader mobi has a wonderful a proprietary text recognition "smart lens" technology that creates a magnification window over any text that you tap.
I personally found the Amazon included "comixology " to be rather useless by virtue of it being a Closed Garden" system.