Are you using Windows? What version of Thunderbird? Pressing F9 doesn't work? Do you see the option from View>Toolbars while in compose window?

I am using Windows version 11 and installed the latest Thunderbird. Contacts side bars do not appear as an option in menu view / Tools - or when I write a new message or reply to sender. It has only malfunctioned in the past week or so. Without the contact side bars it requires me to cut and paste all recipients email addresses into the email. My HP laptop does not have numbered Function keys - it has a row of small icons most of which I do not understand. My cat often walks across my keyboard and upsets configurations. When I open address books, they appear in a different appearance.

I appreciate any hints or help. Les K.

Can you provide a screenshot of the View menu options while in compose window? I am attaching an example of what I'm requesting. That may help.

Does the sidebar and menu work in safe/troubleshoot mode (hold Shift when launching TB)?