When you click reply, forward or write a compose window opens. There is no folder pane in that window. As for missing toolbars, pressing the alt key or F10 should make the menu bar appear. From the menu bar select View-Toolbars and turn the missing toolbars back on.

Thank you for that reply. I take your point about there being no folder pane in compose windows. What has happened is that the little icons/texts saying eg "forward", "delete" etc are now attached to each incoming message, and no longer appear in the main toolbar at the top of the page. How can I get them back onto the main toolbar? Thanks

Those buttons have been located in the message header area for ages. Perhaps you have using a very out dated version? The change occured when Mail Toolbar was moved into Tabs (not above).

As they relate to emails, they are positioned with the emails. So regardless of whether you read the opened emails in the Message Pane, or a tab or a window, those buttons follow the opened email and display in the header area.

You can customise the Mail Toolbar to include those buttons, but as real estate is now restricted due to toolbars in/below tab, you might find it cluttered. But it can be done to suit your own preferences.

View > Toolbars > Customise or Menu con > Options > Toolbar Layout

drag icons onto Mail toolbar. click on Done.