I'm using Firefox and I get the error "Unfortunately this feature is not supported by your browser" when I try to download a mail attachment or a document. What's wrong?

Downloading mail attachments and documents to your local device is not supported in Unusual Suite when using the Firefox browser. This is a known limitation.

To download attachments and documents, please use a different browser such as Google Chrome or Apple Safari. We apologise for any inconvenience this causes.