diagnostic.exe is part of a product called known as XULRunner and the developer is Mozilla Foundation .
If you think that this file contains a virus or another malware, please download a virus scanner.
If diagnostic.exe is using too much CPU or too much memory in your system, it is possible that your file has been infected with a virus. Installing an antivirus is recommended.
Not all processes can be uninstalled without the help of a 3rd party tool, especially malware. To see if it can be uninstalled, first go to Start > Control Panel > Add/Remove programs and select XULRunner from the list to remove the process diagnostic.exe from your computer.
If you have any problems with diagnostic.exe, you may try using a registry cleaner or a speed-up software to check, analyze, and fix problems that are affecting the performance of your computer.
You may try contacting Mozilla Foundation, the developer of diagnostic.exe and ask for more information.