Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Thanks Matt, but have tried that in the past .. anyways have just tried it again now but doesn't help.

Forgot to mention, I can't add any addresses to the address book manually. There are a few names in there ATM but only has an email addr specified. That email addr autocompletes when composing new mail.

Automatically add email addresses .. dropdown still has nothing

Sorry correction to the above ... "ATM but only has an email addr specified" .. I meant "ATM but only one entry has an email addr specified" the others are just names

without email addresses, the auto complete can not work. So how about adding some email addresses to your address book.

Thanks again Matt, I have tried to add addresses to the the address book but want save it. The address book was populated earlier but most entries have disappeared, maybe when I did a Thunderbird update.

Now if I create an entry, populate the fields and click on OK, it does nothing and only option I have then is to click on CANCEL.

Are you trying to use the native OSX address book? Thunderbird can't write to that one.

Thanks Zenos, No I have created a new address book in Thunderbird but want let me write to it.