Opening files in browsers is something that Chrome handles a little better than Firefox but the great thing about Firefox is that there will always be an extension that will replicate the best features in just about any browser and make it available for Firefox users. Open Document in Google Docs Viewer is an extension for Firefox that lets you open PDF, PPT and MS Word files in Google Docs from the right-click context menu.
Once installed, the extension adds a Open Document in Google Docs option to the right click context menu. Click on any link to a PDF, PPT or Word file and click the option. The extension even works on files in Google search results.