Adding iCloud mail to Thunderbird

As far as I know selecting that checkbox only works for Outlook contacts, Calendars and Tasks. It's not possible to sync Thunderbird with iCloud without using another 3rd-party app for that. You can add other email accounts (@gmail, @yahoo, etc) in the Apple mail app on iPhone/iPad and sync your other email accounts that way. You would also have to import your contacts and sync your calendars separately though.

If you just want to check your @icloud email using Thunderbird you might run into some problems if you have two-step verification turned on for your Apple ID.

If you do not have two step verification enabled on your Apple ID, you can safely ignore this note.

If you use third party clients such as Microsoft Outlook or Mozilla Thunderbird to manage your iCloud Mail account, you will need to generate an app specific password.

To generate an app specific password go to http://appleid.apple.com

Sign in with your Apple ID, then click "edit" next to "Security"

There you will find the "generate app specific passwords" link

I have not used Thunderbird, but you will absolutely need to create an app specific password for it to access your iCloud account. That will require either Apple's 2 Step Verification or 2 Factor Authentication be enabled on your Apple ID.

Using app-specific passwords - Apple Support

Two-factor authentication for Apple ID - Apple Support<< requires an Apple device

Two-step verification for Apple ID - Apple Support << does not require an Apple device

I used to have a specific password for Thunderbird for my iCloud mail but I just set up a new iMac with High Sierra and it isn't recognized in the Thunderbird settings for iCloud imap. I was thinking I need to set up a new one for the new computer.

I tried going to appleid.apple.com Security and clicked Edit but there is no "generate app specific passwords" link. Has that moved to someplace else?

I used to have a specific password for Thunderbird for my iCloud mail but I just set up a new iMac with High Sierra and it isn't recognized in the Thunderbird settings for iCloud imap. I was thinking I need to set up a new one for the new computer.

I don't have Two-factor verification turned on. I tried going to appleid.apple.com Security and clicked Edit but there is no "generate app specific passwords" link. Has that moved to someplace else?

Another answer said two-factor verification needs to be turned on to get an app-specific password. What is correct?

Is it possible to add it to other Mail Clients, for me - Mozilla ThunderBird?

Yes.

Not though iCloud for Windows, but independently.

Ignore my reply to reply to @RainbowAimee. I meant to reply to appluser18 but cannot edit or delete the reply.