The coloured bars are Thunderbird's normal way of handling quoted text. I doubt they have any effect on initial cursor placement.

I have seen the bars replaced by chevrons when a quote is returned to me by another email client. I don't know why Thunderbird doesn't recognize it as quoted text and use the normal bars.

There is a config editor setting to disable the bars and use the chevrons, but IIRC this applies only to plain text messages, where the chevron is a de-facto standard.

Paolo Kaosmos's freeshell.de site has couple of add-ons for tweaking quotes and their formatting. These allow you to disable the bars (and chevrons) altogether, using instead the Microsoft convention of having no indentation, just separating successive messages by horizontal rules.

Thank you for your response.

I recognise that the coloured vertical bars are TB's default way of defining quote levels. Until this recent occurrence I've normally seen chevrons, only occasionally the bars. The changes to bars and to 'Reply below quote' have happened at the same time. So in that sense at least they are connected. Something has occurred to change them both.

I'm not too worried by the bars themselves, I'd rather have my 'Reply above quote' back.

Confirm the account settings. Right click on mail account name in Folder Pane and select 'Settings'. select 'Composition & Addressing' for the mail account

Do you have the following selected ?...see image below Select: 'Automatically quote the original message when replying' select:..then 'start my reply above the quote'

You would need to select this for all of your mail accounts.

click on 'OK'

If you already have the correct settings: Please check to see what happens if you test in Thunderbirds 'Safe Mode' In Thunderbird 'Help' > 'Restart with addons disabled'

Test to see if the 'Reply above quote ' now works correctly. If it works correctly then it is likely one of your addon extensions is causing a conflict.

Start thunderbird as normal and disable one addon at a time, restarting and testing to see which one is the culprit. If you locate an addon causing an isse, it may need updating or perhaps it is no longer being maintained. Please report back on your findings.

Thank you for your suggestions. I have already checked all my identities (several times!), and have just done so yet again. Safe mode makes no difference (sadly).

I have discovered that when I reply to SOME e-mails TB behaves as set up, but not with others. I am trying to work out what the difference between the e-mails is. It is not as simple as HTML/plain text. I will report back if I can find the determining factor.