Mozilla Thunderbird review

Thunderbird earns the top spot because it is the one free app here that answers both the Windows and the Linux question with the same program. Mozilla's download page offers 64-bit and MSI installers for Windows, 64-bit and 32-bit builds for Linux, a macOS version, and even builds for Windows 7 and 8.1. Inside, calendars, messages and contacts share one window, so an emailed invitation and the event it creates live side by side. You can work with several accounts in a unified inbox or keep them apart, and tags, filters and layout options let you shape it to your routine. The project is open source and funded by donations, and Mozilla says it does not sell ads in your inbox or collect personal data. The trade-offs are about focus. Thunderbird is a mail client first, so someone who only wants a calendar may find it heavy, and there is no booking page for clients. For a free, cross-platform calendar, it is the safest pick.

Who it is for

Anyone who wants one free desktop calendar that works the same on Windows and Linux.

Price

Free, funded by user donations.

Free forever. Donate to make it better.
Quoted from the vendor pricing page on November 10, 2026.

Key features

• Messages, calendars and contacts in one desktop app
• Downloads for Windows 64-bit, Windows MSI, Linux 64-bit and macOS
• Unified inbox or separate views for each account
• Tags, filters and customizable layouts
• Open source and funded by user donations

Strengths and weaknesses

• Strength: Free with no paid tier
• Strength: Same app on Windows, Linux and macOS
• Strength: Still offers builds for Windows 7 and 8.1
• Weakness: Calendar sits inside a mail client, which is more than some people need
• Weakness: No built-in booking page for clients
• Weakness: Mobile app is Android only, with no iPhone version

Key facts

• Price: Free
• Desktop platforms: Windows, Linux, macOS
• Mobile: Thunderbird Mobile for Android
• License: Open source

Where it ranks

Mozilla Thunderbird is No. 2 in our Best Desktop Calendar Apps verdict.