This extension is fine. It does a bunch of stuff. I love Windscribe and think it's a great option for a light-use VPN.
That being said, it is just better to get the desktop version of the app instead. The only way I would justify choosing the extension over the desktop app would be if you were using a portable version of Firefox. Otherwise, get their desktop application.
Secondly, I was having an issue where Firefox was starting up and then not loading my home page (www.google.com). At first I blamed it on uBlock, but then I did some debugging and found that this extension blocks the first page from loading when Firefox starts. My guess is it has something to do with either the extension refusing to initially load the page, or if Firefox doesn't want to load a page on startup until all extensions are loaded. Either way, I'd rather remove the extension than use a different browser. :(
Edit: After reading further reviews, it has been made clear by Windscribe that they have only put priority on the Chrome extension and have not been updating this problem despite its being reported on their subreddit.