There is so much it really cannot be explained in one go. I have include various links but please note that not all have been updated to the 78* version as this is reliant on community helping and there is a lot to update. So some images may look not quite correct, but the processes are still available, you can always ask a new question on any specific point to get direction. There are a load more Help Articles - I could not include all :) But... a 'profile name' folder contains all the data relating to mail accounts (email addresses), contacts, filters, preferences etc. A default one is created the first time you run Thunderbird. Usually, most people only need and use one profile, but your are not restricted to only one. https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-thunderbird-profiles Each 'profile name' folder can contain many mail accounts. Each mail account is created for each email address. Each mail account can be set up as a POP account or an IMAP account. Info on IMAP Synchronising: https://support.mozilla.org/en-US/kb/imap-synchronization There is also a default special mail account called 'Local Folders'. Thunderbird does offer OAuth2 Authentication for gmail and yahoo. Please read this info all about 'compacting' - this is qute important to maintain a healthy Thunderbird. https://support.mozilla.org/en-US/kb/compacting-folders Each mail account can be set up with Message Filters. They operate automatically on any incoming mail that is downloaded to the Inbox. But can also be run manually on any selected folder. https://support.mozilla.org/en-US/kb/blocking-sender There are various types of 'Search' within Thunderbird including a Quick Filter, Global Search and Search Messages. Various privacy and security options from blocking remote content to OpenPGP https://support.mozilla.org/en-US/products/thunderbird/privacy-and-security-settings The 'Address Book' has two default address books called 'Personal Address Book' and 'Collected Addresses', but you can create more address books. Each address book can have sub groups -'Mailing Lists' - which can be used to perform group mailing. https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird There is a built in 'Calendar' where you can create 'Tasks' and 'Events'. https://support.mozilla.org/en-US/products/thunderbird/calendar News Feeds and Blogs and Chat (please note Twitter is no longer offered) https://support.mozilla.org/en-US/kb/how-subscribe-news-feeds-and-blogs https://support.mozilla.org/en-US/kb/instant-messaging-and-chat This article has a lot of info on composing emails: https://support.mozilla.org/en-US/kb/addressing-email This link has many useful tips and helpful info: https://support.mozilla.org/en-US/products/thunderbird/tips-and-tricks There are a few ways to easily customise your Thunderbird, you can choose to display the good old 'Menu Bar' at the top, layout, choose fonts, themes, etc. Thunderbird can also be manually tweaked by using a 'userChrome.css' file or modified using 'Addon' extensions, but the extensions are written by 'authors' who are not employees of Thunderbird and they do ask for donations to help maintain their addons. That should get you going, but really it is best to learn as you go and ask questions on specific topics if required.