The problem you are having seems to be with the add on it self. Unless you have tried it on other browsers and it worked, If so please let me know and I will further help you.

Thank you for contacting Mozilla Support.

GamerFreak said

The problem you are having seems to be with the add on it self. Unless you have tried it on other browsers and it worked, If so please let me know and I will further help you. Thank you for contacting Mozilla Support.

No, the problem is with the Westpac add-on for Firefox, it is not needed for IE or other browsers. I tried it again and still get the same message that it downloads but will not install because the add-on itself is corrupt. Once I acknowledge the message it disappears and there is no download to check.

Please contact Westpac support for help with their add-on for Firefox.

This happens if an extension is signed. It is possible that your security software has scanned the XPI file v(ZIP archive) and has broken the file by doing this.

Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading and installing work.

cor-el said

This happens if an extension is signed. It is possible that your security software has scanned the XPI file v(ZIP archive) and has broken the file by doing this. Try to disable the real-time (live) scanning of files in your anti-virus software temporarily to see if that makes downloading and installing work.

Thanks for the tip, but in the end I resolved the problem by disabling "Always use private browsing". Simple really and my fault for not remembering the check my own settings!