If you prefer Microsoft Viewer use this add-on:
https://addons.mozilla.org/firefox/addon/online-docs-viewer/

To view pdf files with the Google Viewer use this add-on:
https://addons.mozilla.org/firefox/addon/google-pdf-viewer

My other add-ons:
https://addons.mozilla.org/firefox/addon/always-preview-webextension
https://addons.mozilla.org/firefox/addon/viewerjs-we
https://addons.mozilla.org/firefox/addon/docs-online-viewer-fork/

Install

• License: Mozilla Public License 2.0
• Weekly downloads: 37
• Average daily users: 1687
• Rating: 3.5882/5 of 17 ratings
• Created: 2017-09-21 19:44:44
• Last updated: 2018-05-18 21:00:06
• Homepage:
• Support site and email
• Orig: https://addons.mozilla.org/en-US/firefox/addon/google-docs-viewer/
• API: {a734ba68-4aac-41e0-9141-9f8d00373d93}