I have no idea who is taking the time to astroturf MetaMask on the Firefox review page, but just go read how ridiculous and poorly-written some of these one-star reviews are before making your own judgement.

MetaMask is excellent and does a real service for the Ethereum community. The developers are quite responsive, just go ping them on their GitHub if you ever run into problems with the extension. If there's something inherently unsafe about MetaMask, then it's something that the eyes of 122 contributors and countless auditors have all missed.

Good

The extension or a website the javascript code is injected to by metamask tried to connect to shapeshift.io which is rated "poor" by talosintelligence.com. The connection request is made approx. 60,000 times a day. The rejected connection request is not reported to the end user. The issue attracted attension to the network administration in our LAN.

Its very complicated and doesn't explain or give good directions

I really don't understand what is wrong with this extension. At first I thought it wouldn't work with Firefox Nightly or Dev etc. So I installed the stable version of Firefox with the latest update and this extension still doesn't open. It works the very first time after install and I can restore my wallet with my mneumonic phrase but, after that, it never works again. It causes more grief and is a waste of time, plenty of other serves offering very similar if not, much simpler and safer options.