Pretty good alternative to the full client. If you need to quickly access that unreachable website, or need to dive into a true incognito mode, using this plugin is quicker than the full client. It also doesn't disturb connections from other programs. Also you can install this on a machine where you can't get elevated privileges (this goes for Windows, but possibly macOS and Linux as well).
This plugin does have some quircks. The popup disppears and clears the login fields when Firefox is defocused, so you can't copypaste login information from a password manager.
Secondly, after clicking the icon it takes a second or two for the popup to fully load, which I don't understand - why can't it display itself instantly?
Lastly, how can we be sure the login information is stored securely?