Using New Outlook on Windows for personal email, calendar, and contact management
Hi, kevin eldred
Good day!
Thanks for posting into our community.
According to your description, I understand how frustrating it can be when things don't work the way you're used to.
Unfortunately, Outlook for Microsoft 365 doesn't support opening emails in new tabs like Thunderbird does. Outlook opens emails in new windows by default, and there isn't a built-in option to change this behavior.
We truly understand this brings a lot of inconvenience for you. Since it is approaching into the design level that for our community team that we could not do much for you. At this moment, would you please share your precious idea and suggest it with Microsoft Outlook's engineering team directly -
Contact support and provide feedback in new Outlook for Windows - Microsoft Support.
In addition, you can try managing your open windows more effectively by using the taskbar or the Alt+Tab shortcut to switch between them quickly.
Thanks for your precious time and your understanding would be highly appreciated.
Hope you all the best!
Microsoft Community Moderator.