Profiling Thunderbird Performance

Creating a performance profile can be useful for developers to find the causes of high CPU load or slowness in Thunderbird.

Preparing performance recording in Thunderbird

The performance recorder is part of the Developer Toolbox in Thunderbird. It is not shown by default, so you need to enable it first from the toolbox settings, as described below. Before you start your recording, prepare the scenario which you want to test in Thunderbird.

1. In Thunderbird’s main window, open the Developer Toolbox:
Press Ctrl + Shift + ICommand + Shift + I, or click on > Tools > Developer Tools > Developer Toolbox.

2. Important: Confirm the warning prompt Incoming Connection with OK so that the toolbox can connect to your Thunderbird application. Note: The prompt might be hidden behind other Thunderbird windows.

3. In the new Developer Tools window, open the Settings:
Press F1, or click on the ••• button in the top-right corner, then on Settings.

4. Under Default Developer Tools, ensure that [✓] Performance is checked.

5. Click on the Performance tab that has appeared at the top of the toolbox.

6. Important: Before starting your performance recording, go back to the main Thunderbird window, and prepare it so that you are ready to record the performance of the scenario that is slow or causing high CPU load.

Recording and saving a performance profile

You are now ready to record and save the performance profile of your scenario. Unless told otherwise, try to keep your recording as short as possible by recording only the exact set of actions which are suspected to cause the problem.

  • In the Performance tab of the Developer Tools window, click on the Start Recording Performance button.
  • Without delay, go back to the main Thunderbird window and perform the set of actions you want to record.
  • Once finished, immediately select Stop Recording Performance in the Developer Tools window.
  • On the left side, right next to the new Recording #1 item, click the Save link and save the file to somewhere on your computer. The file extension must be .json.

Sharing a performance profile

1. Go to (yes, on

2. Select Load a profile from file and select the .json file which you just saved for upload.

3. Important: Near the top-right, select the green Publish button, then in the Share Performance Profile panel, select the (blue) Publish button again.

4. Copy the link (e.g. which pops up in the top-right corner under the new permalink button and paste it into an appropriate new bug or existing bug on Bugzilla (BMO).

5. Finally, back in the Developer Tools Performance tab, you can delete the performance profile using the trash-icon just above the profile list in the left-top corner (where you saved the profile), and then close the Developer Tools window.

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