Setting Gmail as your default mail application for email addresses and links within Firefox is rather simple nowadays. Previously you had to dig deep into the about:config preference system but now the configuration is present within the user interface.
Access the Firefox Options through the menu/navigation system in the browser.
Click the Applications tab and within the Search field locate the mailto content type, adjusting the Action type to Use Gmail. Now any clickable email addresses will initiate a Compose window in Gmail with the address already inputted.
While still on the Applications tab, to configure Google as your default RSS reader search for the content type of web feed and adjust the Action to use Google – just as you did above. Future access to RSS feeds will allow you to select either the Google Personal Home Page or Google Reader as your content reader.
Unfortunately, at this time Firefox does not have the capability to view or edit the document types of PDF, DOC, XLS, etc directly into Google Docs. However the strength of Firefox extensions are made evident again in this instance. Open Document in Google Docs Viewer is a no frills add-on that adds the Google Docs option in the Firefox’s right-click context menu on links with the appropriate document type.
Unlike many extensions, Firefox requires no restart to complete the installation. Instead, any time a PDF or DOC file is linked to from within Firefox simply right-click and choose to Open Document in Google Docs. How simple is that?