[rhelmer]

WebExtensions are a superset of what Chrome does. Things like tree-style tabs will be possible in Firefox with the new extension system, for instance.

There are already several WebExtension APIs implemented that Firefox supports that Chrome does not, the difference now is that APIs are being built specifically for extension authors to use, rather than Firefox internals being directly accessible (which certainly allows you to modify anything, but is intractable to maintain backwards compat and secure).

Many authors of popular extensions are working on specifying and prototyping the APIs they need.