This answer will explain the process you should use to receive email from your IMAP mailbox using Mozilla Thunderbird.
Set up your emails in Thunderbird video guide
If this is the first time you have accessed Mozilla Thunderbird, it will automatically start the Mail Account Wizard. Enter your name, email address and password then click Continue.
Tip: If this does not happen then click on the Tools menu, then click the Account Settings option and click the Account actions option and then click on Add Mail Account.
If you receive the message
Thunderbird failed to find the settings for your email account
Select IMAP from the Incoming drop down box and input the Incoming and Outgoing server details manually.
The Incoming server setting is imap.123-reg.co.uk
The Outgoing server setting is smtp.123-reg.co.uk
Make sure to input your full email address into the Username field
After manually inputting the server settings click the Re-test button.
This will auto fill the Port, SSL and Authentication fields.
Click OK to finish adding your 123-webmail account to Thunderbird.