Firefox can handle PDFs and images itself, but most other kinds of email attachments require another application.

What kind of file was it? For example, was it a word processing document (for example, a .doc or .docx file) you might normally view/edit with Microsoft Office, Google Docs, or a locally installed program like OpenOffice or LibreOffice?

It is a docx file. I would like it to open with LibreOffice. How do I reset the default there?

It depends. If you are clicking a button that sends a download directly to Firefox, then Firefox would check the Applications list, which you can update on the Settings page. See: Manage file types and download actions in Firefox.

If you are clicking a button that shows a preview or takes you to Google Drive, then hopefully there is a different button you can use to get a direct download.