You'd better ask at the Thunderbird for Android channel. https://support.mozilla.org/en-US/questions/thunderbird-android

Thanks. Haven't yet got my head around having 2 sorts of Thunderbird!

Just a quick question - why can't I find this forum if I Google Thunderbird for Android support or forum or if I go to Thunderbird help?

In Thunderbird In 'Quick Filter' toolbar The icon which is 'Envelope with a dot in top left corner' is 'Show Only Unread Messages' filter option. You would select it to only show unread emails and delect to show all emails.

If using 'Table View' you will see column headers. One column header shows an 'Envelope with a dot in top left corner' and if you click on it - it means 'Sort by Unread' and each email will have a green dot in that column if status is 'Unread'.