You may think that Google considered Docs & Spreadsheets important by integrating import files or export files functionality to it, but wait, you can do that one file at a time, and that becomes tiresome if you have a lot of files to import or export to.
To overcome this problem, the latest version of Google Toolbar for Firefox has a feature that lets you drag and drop files to your browser and they are automatically uploaded to Google Docs. To upload more files at a time, go to Windows Explorer or other file manager, select your files and drag them in Firefox. To activate the feature, to go to Settings and enable "Docs & Spreadsheets".
Source:→ Google SystemGoogle, Google Docs, Google Toolbar, Firefox