Thunderbird 60 will be the next major release of the desktop email program. It is a major release because Thunderbird is moved to a new Extended Support Release base (and as is Firefox when version 60 hits).

Thunderbird 60 comes with a refreshed user interface and plenty of other changes. Thunderbird users who run extensions in the email client may face compatibility issues as Thunderbird uses a strict compatibility policy when it comes to extensions.

In plain English: any extension that is not explicitly compatible with Thunderbird 60 will be disabled. Attempts to install incompatible Thunderbird extensions are blocked and users get the "could not be installed because it is not compatible with Thunderbird" error message.

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The change is disastrous for Thunderbird's extension ecosystem. If you check the first two pages of extensions that have the most users on the Thunderbird Add-ons site, you will find out that only eight of the 40 Thunderbird extensions with the most users are compatible at this point in time.

The situation is even worse for the top 40 add-ons with the highest ratings as only five of them are compatible with Thunderbird 60.

Tip: you can verify if your Thunderbird extensions are compatible by visiting the Add-ons site and searching for the extensions.

There is a way out, however, at this point in time. The Thunderbird development team added an option to opt-out of the strict extension compatible checking enforcement. While that does not necessarily mean that extensions will work if they are not listed as compatible, chance is good that they do if they have been updated recently.

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Here is what you need to do to flip the switch and disable strict extension version checking in the email client:

1. Open Thunderbird if it is not open already.
2. Tap on the Alt-key to display the menubar and select Tools > Options from the menu. Alternative: click on the new menu icon (the three horizontal bars) and select options when it opens.
3. Switch to the Advanced tab.
4. Click on the Config Editor button under General (it opens by default).
5. Confirm that you will be careful.
6. Search for extensions.strictCompatibility.
7. Double-click on the preference.

A value of true (default) blocks extensions that are not explicitly listed as compatible with Thunderbird 60, a value of false disables the strict checking.

Thunderbird 60 is available as a beta version at the time of writing. You could install it on another machine or a virtual machine to test extensions that you require. The new version of the email client will be released on May 15, 2018 to the stable channel. Thunderbird 60 will be released a week after Firefox 60 which Mozilla plans to release on May 9, 2018.