what about right clicking any folder in the folder pane and selecting the open in a tab option. It is not just restricted to accounts.

No, that just opens a tab with the standard email view, not the Outlook one.

Just a wild guess: maybe you have one of the following add-ons installed in Thunderbird on your laptop, and you clicked on some links, so they opened in tabs:

1. Open Tab https://addons.thunderbird.net/en-US/thunderbird/addon/open-tab/

Quotation from the above link:

Link open in new tab.

2. Link in Tab https://addons.thunderbird.net/en-US/thunderbird/addon/link_in_tab/

Quotation from the above link:

Link in Tab allows you to: - Left click => open link in new tab - Left double-click => open link in browser - Middle click => open link in browser and close current tab

3. BrowseInTab https://addons.thunderbird.net/en-US/thunderbird/addon/browseintab/

Quotation from the above link:

BrowseInTab lets you open and browse any link in a Thunderbird content tab. Links are followed within the tab and back/forward navigation is supported. Also with enhanced Zoom customization and a url bar.

Thanks for the ideas, but no, I don't have any of those add-ons. The only add-on I have in Thunderbird is OWL. As far as I know, I have never explicitly opened these Outlook tabs. I don't know what made them appear. In fact, I cannot see any way to open a tab other than "Open in New Tab" option on the popup menu on the navigation tree, which just opens the same view as selecting the item, and Recently Closed Tabs on the Go menu of the menu bar or the popup menu on a tab header, which let me reopen one of these tabs if I've closed it. Other actions open tabs, e.g. Search, and selecting a found email, but I don't see any way to just open a tab.