I'm running Windows 8.1. I used Thunderbird about a year ago on the same laptop, no problems with the calendar. I had to reimage this computer a few months ago. Now I have reinstalled Thunderbird and all my email accounts work fine, including my Gmail. But as soon as I went to the Calendar tab I was prompted for my Gmail login info. Numerous attempts later, the email connects but calendar login is refused. I went to Tbird help and followed all the suggestions there, no luck. I installed Lightning and the Provider add-on, then the calendar disappeared totally from Thunderbird, even the icon in Options disappeared. Uninstalling the add-ons made no difference. I had to un/reinstall Thunderbird to get the calendar back, then I reinstalled the two add-ons - no luck. I went to the Gmail site and confirmed my server and other settings were already as described in the Tbird help - weaker security apps permitted etc. I followed all suggestions on the Google calendar help page, nothing worked. I even tried sharing my Google calendar with one of my other email accounts. I was able to access my calendar after that, but in read-only mode because it was shared. Then the Provider add-on blocked that access, so I'm right back to square one after 2 days of frustration. I'd appreciate it if someone could suggest some way for me to get Thunderbird to connect to my Google calendar so I won't have to use the browser interface. I've used other email clients to connect with no problems, most recently yesterday. If it's not possible due to bugs etc. please let me know so I can find another email program.