Can you please try to uninstall again after killing all the process named "firefox.exe" from your task manager and running the uninstaller again ?
If it uninstalls successfully, kindly perform a fresh installation again by downloading offline installers from Installation problems | Flash Player | Windows located under "Still having problems" section at the bottom of the page.
Before Uninstalling, Can you please ensure no Firefox process is running in the back-end.
2) Find Firefox Process, and End the Process and Try Uninstalling Flash Player again.
This happens sometimes, you have your application closed yet an instance of the same process keeps running in the back-end but is not visible on the UI.
Hope this Helps.