Thunderbird review

Thunderbird is the answer for anyone who wants a calendar that is free with no upsell at all. It is an open source email app from the Mozilla family that keeps messages, calendars and contacts in one desktop app, and the project says it is free forever and funded by donations. Thunderbird states that it does not collect personal data, sell ads in your inbox or train AI on your conversations. It handles several accounts separately or in a unified inbox and can be customized heavily with add-ons and layout settings, and Zoho lists Thunderbird among the calendars it syncs with. The trade-offs come from its design. It is an email client first, so the calendar is one panel among several, and it is built for desktop computers rather than as a phone calendar. Setup takes more steps than signing into Google or Apple. For a privacy-minded desktop user who wants nothing to pay, ever, it fits well.

Who it is for

Desktop users who want a completely free, ad-free calendar with no account lock-in.

Price

Free forever, funded by 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
• Open source and funded by user donations
• No ads in the inbox and no selling of personal data
• Customizable layout and add-ons
• Unified inbox across several accounts

Strengths and weaknesses

• Strength: Completely free with no paid tier
• Strength: Open source, with no ads and no data collection for advertising
• Strength: Works with several accounts in one app
• Weakness: Calendar sits inside an email app
• Weakness: Built for desktop first
• Weakness: Setup takes more steps than Google or Apple calendars

Key facts

• Free plan: Yes, free forever
• Paid tier: None, donations only
• License: Open source
• Organization: Part of the Mozilla family

Where it ranks

Thunderbird is No. 10 in our Best Free Calendar Apps verdict.