Steps to reproduce the issue

Write a component with an installer scipt that results in an installation error e.g. compile error, etc... Pack that component into a zip file and upload it with firefox using the joomla extension manager. You will get a PHP error message e.g. compile error and ofc. the component won't be installed.

Expected result

The zip file will be unlocked after the upload.

Actual result

The zip file is locked by firefox until it is closed.

System information (as much as possible)

Win7x64 SP1, Firefox 51.0.1 (32 bit), Joomla 3.4.3

Additional comments

By successful extension installation the file is unlocked properly, so there should be something in the code which keeps the file locked by unsuccessful installation. I am not sure whether the bug is in Joomla, or PHP or Firefox.