I have read a lot of articles regarding WebExtensions and Add-on SDK.
It's noted that WebExtension API advatanges includes compatibility across various browsers.
However, my questions are:
Is Add-on SDK platform dependent (Firefox)?
Yes. Note that it's also supported on Firefox for Android, which is currently one of the very few ways to get an extension running on mobile.
Will Add-on SDK continue to be supported in the future?
We can only speculate how long the support would be. Active development of the Add-on SDK stopped, and Mozilla is focusing on developing WebExtension API.
Quote from documentation:
There are currently several toolsets for developing Firefox add-ons, but WebExtensions will become the standard by the end of 2017. The rest are expected to be deprecated over the same period of time.
So according to this quote, Add-on SDK is being deprecated in a timeframe of a year.
Note that both extension models offer different features: WebExtensions are unlikely to ever cover all functionality of Add-on SDK.
Finally..
WebExtension API advatanges includes compatibility across various browsers.
Not strictly true, since extension models are only partially compatible between Chrome, Firefox and Edge. At least fow now, you'll need platform-specific workarounds and some features are either not yet implemented or not going to be implemented in all platforms. Still, it's possible to have a unified codebase.