Rated 4 out of 5 stars

Useful, well working plugin. (does what it says)

Some explanation:
"HTTP Authenticated Sessions" means the "HTTP Basic access authentication".
This isn't what most websites use for authentication! (they use cookies instead)
You can recognize a website using "HTTP Basic access authentication" by two things.
1. The login is begin done in a separate login window, instead of some text fields on the website.
2. There's absolutely no way to logout, except restarting the whole browser. That's why this add-on is needed.

Unfortunately it blocks the new e10s (Multiprocess Windows) feature of Firefox.

On Feb 8, 2017, I replied:

"Mozilla has chosen to require that add-ons be rewritten to use their new Web Extensions API; previous APIs such as those used by HTTP logout will no longer be supported. I need to find some time to learn Web Extensions well enough to evaluate how straightforward it will be to rewrite HTTP logout to use it, and at that point I will know enough whether I can rewrite HTTP logout to use it, or abandon the add-on entirely."

Since then, I've looked at Web Extensions and as far as I can tell, it has no equivalent to the XUL nsIHttpAuthManager API that HTTP Logout relies upon. Until Mozilla adds such an API to Web Extensions, I don't have a way to port HTTP Logout to Web Extensions.