With the latest version at the time of this review (4.0.2), it works fine for checking email, but the UI is still broken.
There are 2 major issues.
1: You still cannot access the email with a single click, instead a single click now opens a menu where you must then select the account you want to open. this comes up even if you only have 1 account added to the extension, this you have an extra unnecessary step.
2: The icon for the latest version is completely ruined, it is literally 2.5 times the size of the older version, and number representing the unread mail, covers other content on the screen. Thus if added to a bottom bat (adddon bar) then the number stick out over the bar and covers content on any web page you are looking at. And if you have it at the top, it will cover content from other buttons you may have.

The developer either intentionally ruined the UI, or is for some reason does not care enough to fix these bugs. I know that some changes are needed to work with the new way firefox will handle extensions in the future, but those changes does not mean that we need to have these 2 major bugs. I first reported these issues about 4 months ago, and with new versions, the developer has actually made it worse