Rated 4 out of 5 stars

1. I have installed several dictionaries, but after several attempts this one does not get working. (
Firefox Nightly 49.0.2)

Still does not install. I simply do not get it into my list of languages. For clarity, I am using: firefox-49.0.2.en-US.win32-add-on-devel.installer.1474711644 on Windows 10 Pro Anniversary English.

Sorry, false alarm. I had had a mysterious automatic and wrong update of Firefox with as side effect that a lot of add ons got disabled(?!). When I tried to install this one, it was already there but disabled, without me noticing it.

2. De laatste versie die wel goed installeerde herkende nog steeds (na 9 september) geen samengestelde woorden.
Is nog steeds op niveau van mensen die spatiegebruik.nl niet snappen.

Dit werkt nu ook.

This review is for a previous version of the add-on (3.1.2). 

Zou u nogmaals willen proberen te installeren in Nightly?
Zojuist getest op een verse (niet Nederlandse) Nightly, waarin de add-on wel installeerde en ook bijvoorbeeld "stopcontact" goedkeurde.

The add-on should be able to install up to Firefox 52.
It needs to be shown on the add-ons-page under the tab Dictionairies. You can also test by right clicking on a formfield, check spelling, right click again and see if Dutch shows up under Languages.

Can you also test with a Nightly-version from https://www.mozilla.org/en-US/firefox/nightly/all/ ?

Good to hear it's working properly right now and the upgraded rating!
Goed om te horen dat het nu wel goed gaat en de betere waardering te zien!