It has worked fine until recently.
I'm using Firefox 50.0.2 on Windows 10 and clicking the Camel icon right next to the address bar returns an empty white panel.
Disabling uBlock and NoScript extensions didn't help.
Still, 4 stars because it deserves them!

For the last week or so all I get is "An error has occurred." when clicking the button on an Amazon product page. The same page works fine on Chrome. The other extensions I have installed are Umatrix, Ublock Origin, and Fakespot.

Support has released v 2.9.5 that successfully addresses the blank page encountered by some in v. 2.9.4.

It's a wonderful app, but the newest update broke it. The icon in the taskbar/menu is missing entirely. The app is installed, but there is no way to actually use it right now.
--- EDIT ---
it appears that the icon is in the address bar. I didn't even notice it moved. 5stars