Not quite — if you run Firefox nightly you can develop and use "webextension experiments", which have the same power as the old pre-webextension Firefox addons. If you wish, you can also apply to have your experiment be turned into an official webextension API for Firefox.

This is considerably more cumbersome than previously, but still far better than having to fork the whole browser. (On the topic of forks, if running a bleeding edge browser — nightly — is not for you, you can use Waterfox, which is approximately Firefox ESR + the ability to run full-powered extensions.)