As official doc said, addon-sdk will be deprecated.
But seems like WebExtension doesn’t support devtool dev now.
There will be a hell of a lot more useful add-ons being disabled when Mozilla actually drops support for non-WebExtensions in Firefox 57, missing APIs don’t seem to be a valid excuse to Mozilla.
If you want to start not, you can probably do so in some Chromium based browser. The final APIs should be at least similar.
You can find out more about the devtools status on http://arewewebextensionsyet.com/ and it is indeed not finished yet. With the add-on SDK you can do bascially anything, but the APIs for interacting with devtools will not be the same, they will be APIs specific to Firefox.
I’d suggest to either wait until the devtools apis are available, or go with an embedded WebExtension and defer anything that is not implemented yet to the addon sdk.
Probably even better, you can also work with a WebExtensions experiment and implement the devtools APIs you are missing on your own. This may actually help the Firefox team with providing those APIs. I’d suggest to drop by on irc.mozilla.org in the channels #webextensions and #devtools.