Incompatible extension - Thunderbird
From MozillaZine Knowledge Base
If you get an "incompatible extension" error message while trying to download/install a Thunderbird extension, it's probably because you left-clicked on the download link while using Firefox. Firefox and Thunderbird both use extensions with an xpi file extension, so Firefox can get confused and think you're trying to install a Firefox extension. It's only after it actually tries and fails to install it that it realizes it's not a Firefox extension. Unfortunately it's not smart enough to tell you that the real reason it failed is because it's a Thunderbird extension.
You need to right click on the download link and choose "Save link as" if you're trying to download it using Firefox. This problem doesn't occur with non-Mozilla browsers such as Internet Explorer because they don't know how to install .xpi files.
Left clicking on "Install Now for Windows" for the Lightning extension displays a Software Installation window in Firefox. If you press the "Install now" button it displays a Add-ons window plus a "Incompatible extension" popup that states:
What you should do is:
You need to restart Thunderbird to use the extension. Occasionally you have to do this several times before it installs the extension. Then go back to Tools -> Extensions and see if you need to enable the extension, or use the options button to configure it. If the extension adds several buttons (such as the Buttons! extension) they're available but won't be displayed until you customize the tool bar. Right click on the tool bar and drag and drop the desired button to the tool bar.
Some browsers change the file extension of any files you download. This doesn't occur with Firefox. If it happens to you, rename the file extension to .xpi and then install it.
If Thunderbird (not Firefox) complains that it's an incompatible version, you can use the Nightly Tester Tools Lite extension to override the version check. It adds a "Make all compatible" button. This doesn't guarantee that the extension will actually work, but it frequently works around the problem. However, you are better off looking for a compatible version of the extension. Try to find a link to the authors home page and look for a version history section. That will typically have download links for other versions. You could also search the Extension/Theme Releases forum for a thread about that extension. Many times a author will provide a link to a version of an extension for a nightly/beta build there, while the Mozilla Add-ons or the The Addons Mirror web sites only have extensions for the released version.