Good idea Rated 4 out of 5 stars
I've been going to APOD for a few years now by way of their feed. Having the add-on is a nice "extra". You should, however, add 'background-position:center;' to your CSS--some images are rather "tall" and one is left looking at the very top-most part of them (which typically doesn't show the object of the image). Plus, when the POTD is a video, something other than a blank page as the background would be preferable.
Feature suggestion: hover the left margin to hide the grid (to "reveal" the image--yeah, I'm aware of the toggle button, but that turns it off until you turn it on again; the "hide on hover" is just a mouse-move to the left while the toggle button is a move to the right + a mouse click to hide + another mouse click to restore ...so, simpler to just hover for a temporary action :D )
Hello, thanks for your feedback.
Version 1.4 includes a configurable centering switch. Regarding the feature suggestion, on the top-right side of the page there's a button to hide the grid (That comes from Firefox itself)
I'm still thinking what to do with videos... POTD sites aren't supposed to contain videos.. ;-)