Yesterday I downloaded Thunderbird to Windows 10 (64-bit), added my Office 365 (for work or school) account, rocketmail account and iCloud account. I created unified folders for the three accounts. I have downloaded the TBSync add-on with the required CavDev (?) one to connect my iCloud calendar. After successful use the first few times, now Thunderbird becomes unresponsive within 5 seconds of opening, every time, rendering the program unusable. Troubleshooting mode does not fix the problem, so I assume it's not the add-ons, nor does starting my PC in Safe Mode (F4 at startup ie no networking), so I assume it's not antivirus. I uninstalled Thunderbird and re-installed from the website today. All my accounts were there (so I wonder if the uninstall didn't work properly, although I assume that it's just the files left on my computer that the new Thunderbird installation is accessing). All these attempted fixes leave the problem unchanged: the window freezing within seconds of opening. Because the program becomes listed as 'not responding' and greys out on further clicks, I have to hard-close the program using Windows, selecting 'Close the progam'. Clicking 'Wait for the program to respond' leaves the screen greyed out, and nothing changes for at least 20 minutes (the maximum I've tried waiting it out). This also means that it does not yield a formal crash report, because Thunderbird does not recognise it has crashed. On one occasion it did create a crash report, which I submitted. I am not sure what behaviour allowed this to happen, and I have been unable to reproduce it. That particular crash ID was as follows: Crash ID: bp-41096385-eebe-4ccc-a47e-f76300240417 Any help much appreciated! I'd love to stick with Thunderbird if possible.