Thunderbird Send needs cookies and JavaScript enabled to complete an upload or download. If either is blocked, the transfer fails immediately with no error message, which can look like a random or unexplained failure. Where you enable cookies depends on whether you're using Send in a browser or through Thunderbird itself.
Using Send in Firefox
- Click the menu icon (☰), then "Settings."
- Select "Privacy & Security."
- Scroll to "Enhanced Tracking Protection" and open "Advanced Settings."
- If it's set to "Custom," make sure "Cookies" is unchecked, or switch it to "Standard."
Turning off the per-site shield icon alone isn't enough - that only disables tracker blocking for that one site, not the underlying cookie setting.
Using Send in Chrome
- Click the three-dot menu (⋮) in the top right, then "Settings."
- Select "Privacy and security."
- Select "Third-party cookies."
- Choose "Allow third-party cookies."
Using Send through Thunderbird (filelink)
Thunderbird's cookie setting works differently from the one in Firefox and isn't in the same place:
- Click the menu icon (☰), then "Settings."
- Select "Privacy & Security."
- Scroll to "Web Content."
- Make sure "Accept cookies from sites" is checked and set to "Always."
Once cookies are enabled, try your upload or download again. It should go through normally.
If it still fails, reach out to Support with your file's size and format along with the browser or Thunderbird version you're using.