I'm on Windows 10 using TB 78.8.1 64bit latest release. In 'Personal address Book', I have a 'Mailing List' called: Test This

In a Write window, if I start to type: Test, the auto complete offers 'Test This', which if I select, it is entered correctly as: Test This <Test This> in pill form.

In a Write window, if I type : Test This and then press 'Enter/Return' key, the correct entry is made in a pill form.

In a Write window, if I enable the 'Contacts Sidebar', select 'Personal Address Book', I can see 'Test This'as one of the contacts listed. Double click on 'Test This' puts it into the TO field correctly as before.

In 'Address Book', if I select 'Test This' mailing list and then click on 'Write', it enters all the email addresses of contacts in 'Test This' into a TO field and all are in the pill format.

Which method are you using? Please post an image showing what you see in the 'Write' window. I did note that you are using Linux, so the fact that I can do it in Windows OS may not reflect what you can do in Linux. I also noted you are using 78.7.1, does it still occur if you use the latest release version 78.8.1 ?

My address book doesn't show any Mailing Lists, but under one entry 'Boys', shows a group of addresses (see first screenshot). When I write I can't select 'Boys' for the 'To:' field. If I type 'Boys' the entry show red, indicating it's not recognised (see screensshot 2).

I haven't tackled the latest release 78.8.1 because I can't see how to transfer all the data (addresses, accounts, e-mails and local folders from 78.7.1 to the new version. 78.7.1 is the latest version I have available through the update manager.

The reason you cannot select 'Boys' in the address field is because it is not a 'Mailing List'; it is an address book.

If you want to send an email to all contacts in the address book called 'Boys', then you have a choice of doing the following. Listed in now particular preference order, it depends upon what you want to achive. 1. Create a 'Mailing List' in the 'Boys' address book and put contacts into that Mailing list. Then I suggest you use the 'Bcc' option otherwise you will be allowing everyone to everybodies email addresses. https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird

2. Open Write, enable the 'Contacts Sidebar and select the 'Boys' address book. If you want to add all the contacts then do this: Select first contact to highlight then press 'Ctrl+A' to highlight all of them, then click on the button eg: 'Add to Bcc'.

3. If you want to send one email per person and personalise the email for each then use the addon called 'Mail Merge'. https://addons.thunderbird.net/en-US/thunderbird/addon/mail-merge/ This use a template and a selected address book to send one email person in a TO field. Loads of info at the Mail Merge addon link.

Thanks, your no. 2 solved the problem. I hadn't realised that Contacts Sidebar had become disabled by the last upgrade. Enabling it restored the normal behaviour.