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Here you go!

That is normal in Thunderbird when you are in the compose window. It's showing you the structure of the message to help you insert or edit at the right place.

I wonder why you think you're "previewing" it?

If I double-click a message, it simply opens in a full window or tab for reading. I'd have to explicitly click on "Forward" or "Reply" to see what you're showing us.

What add-ons do you have? We heard recently of an add-on that changed the behaviour under double-click in a way similar to what you are describing.

Think you misunderstood me, when you view email in the preview pane of the main interface everything looks fine but if you double-click to open the message (not click forward/reply) it is viewed as per the screencapture and any hyperlinks within the mail don't work. I have used thunderbird for years and this is the first time a client computer has had this problem. No additional add-ons, just thunderbird with lightning as standard. I don't get why it would show anchorpoints or placeholders in an email as it's being used as an editor!

The 'To' box has been cleared to hide the identity of my client but this is not her composing an email this is her doubleclicking an email in her inbox to view it and follow links contained within the body of the content..

Your image is most definitely the Compose window; it has a "write" button.

So when did doubleclicking a received email end up in composing. The content is not hers, it's from some craft mailshot...

I don't know. I can't reproduce that. I can't find anything to double-click on with the effect of opening the compose window.

Could it be a registry glitch? I've tried a repair reinstall, same result. Should I try an older version of tb and if so where from?

Don't waste your time searching through the Windows registry; Thunderbird doesn't use it.

By all means re-install Thunderbird if you want. But configuration settings are stored in the profile and are not expected to be changed by the process of removing, updating or installing the Thunderbird executable. Most performance glitches turn out to be related to settings and so persist through updates of the program itself. But I don't know of any settings, either in the GUI or in the Config Editor, which would have this effect.

Past versions are available here:

https://ftp.mozilla.org/pub/mozilla.org/thunderbird/releases/

I'm using a beta (47) here, but even if I'd had a version doing what you describe, I probably wouldn't have noticed since I rarely open messages in the window/tab by double-clicking. But a large proportion of the user base seem to use the double-click and I'm not seeing the wave of complaints that you'd expect if it were intrinsic to a recent release.

So, I am puzzled.

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.