I love this add-on so far, but there are a few things I'd really like to see before it can completely replace Momentum, my previous new tab page:
1) Currently, metro tile links are completely broken for me. Instead of a nice link to the location I associated with the tile, I'm directed to "moz-extension://932f06b5-4317-6d43-b710-0dcaaff56c7f/"... which conveniently doesn't exist. It would be nice if I could actually visit the links through here!
2) Metro tiles require me to supply my own icon. I can live with this, but why do that when lots of sites have a favicon available anyway? There ought to be an option to insert the site's favicon (or that should just be the default, with an alternate option to supply my own icon in case the favicon isn't suitable).
3) Metro tiles are all uniform- it would be better if I could resize them to my liking in a non-grid fashion (maybe more of a flow layout?). This isn't incredibly important though, and I could live without.
4) Non-link tiles. Specifically, I'd love to have a tile with the current time and date on it as well as a tile with the current weather (with a nice icon for each weather condition, and maybe even a color based on weather condition to boot!). A search tile would also be nice- that way I could have one for duckduckgo, one for google, one for wikipedia, etc, and I could simply enter my query right on the new tab page and launch the query straight from there.
5) It's nice to be able to supply my own background color or image, but I'd really like to be able to just set the background to a rotating collection like Yahoo/Flikr/Google Images provide, with maybe a set of quality images pre-approved. You could even add the option to simply show images based on current location, which would be very cool.

All in all, a very promising start. I hope you'll consider implementing some of these feature requests, because I really think your new tab page could be amazing.