I use this addon for some time now. I use it for my gmail account and my fastmail account. However, since version 4 the fastmail script doesn't work anymore, even it was made by the dev itself. That's no option for me, so i stick to 3.5.23 But the problem in the old version is that gmail doesn't open and instead you're directed to your google account where you have to login. So i now use 3.5.23 for fastmail alone. If the newer version would improve and that i'm able to use both accounts again (gmail and fastmail) i would give it a higher rating.