I'm sorry about the issue. After opening the Adobe flash player installation file, then just close all Mozilla Firefox websites / windows, and continue.
- If that does not work, after opening the Adobe flash player installation file, then press at the same time the 3 keys Ctrl, Alt, Del on the keyboard, then open Task Manager if shown, then click on the Processes tab, then select the item firefox.exe there, then end task it and confirm, then continue.
Please reply at the box below to continue troubleshooting. Thank you.