Well, Manjaro is based on Arch but Arch users don’t take kindly to Manjaro users asking for support in communities closely related to Arch.
Anyways this is what you should install: aur.archlinux.org/packages/thunderbird-bin

iirc you can run pamac build thunderbird-bin. Remember using AUR comes with a few risks: wiki.manjaro.org/index.php/Arch_User_Repository#O…

• Multiple versions of the same packages.
• Out of date packages.
• Broken or only partially working packages.
• Improperly configured packages which download unnecessary dependencies, or do not download necessary dependencies, or both.
• Malicious packages (although extremely rare).