Affected versions

• Firefox 68.0b5
• Firefox Nightly 69.0a1

Affected platforms

• Windows 10 (x64)
• Windows 8.1 (x86)
• Windows 7 (x64)

Steps to reproduce

1. Start two instances of the stub installer at the same time.
2. Observe the behavior while progress is being made on the two installers.

Expected result

• One of the installer will inform the user that a installation is already in progress and have the option to close it.

Actual result

• Both are installed at the same time, which causes the browser to be corrupted.

Regression range

• Will determine if this issue is a regression.

Additional notes

• Firefox will get partially installed, with the desktop icon being broken (a Missing shortcut message will eventually appear during the installations) and will appear broken as well in "Uninstall or change a program" from windows.
• The only way to be able to uninstall Firefox, is to run one instance of the stub installer, let it finish, then it will be overwritten with a working uninstall.