Particularly in error cases we still should delete the temporary XPI.

How do you determine whether an .xpi file is only temporarily downloaded? I archive software that I install on my PC. I download .xpi files and install them from my hard drive. Then I archive them to a flash drive. I don't want any .xpi file deleted unless I explicitly delete it. This also applies to .mar files for incremental updates of Thunderbird and SeaMonkey. I download them, install them from my hard drive, and then archive them.

This doesn't apply to anything you download manually, only to stuff we download to the temporary directory in the process of installing through the UI. We also already implemented tests for this so this is fixed. test_install.js does downloads with failures and we test the tmp directory is empty at completion.

As long as we don't know the exact patch it's WFM. I don't see a reason for a manual test.