I'm going to admit that I was wrong, all apologies. After restarting my browser, it works without logging in. I believe this had something to do with me being already logged in to another google service.
Not sure why but other file types (like .doc) do indeed appear not to work. I get a message on google docs page that this type of file is not supported. I don't know if that is a bug or not. Also... would be more helpful if I did not need to search in context menu. Perhaps a popup to choose to preview or bypass. Or even optional setup to just use the extension as default behavior.
I was actually expecting more of an addon with such a title... like doc, docx, xls, ppt... support. No complains but only pdf is supported, its a bit too short, if more formats were available this could be a real competitor to OpenIT - that you know it will never support PDF because of the deal Dennis Redmontini did with Nitro PDF. Like i said, needs to support all the formats Google Docs supports (and i hope it will), no complaints on coding.
Unfortunately, this add on requires a google docs account, even though it's possible to use google's viewer without one (http://docs.google.com/viewer). There are other add-ons that use the viewer without requiring a login, such as gDoc, however, that addon makes the links always open in the viewer, instead of adding a right-click menu item.
Version 1.0.2 has been working well for me. Note that Firefox's check for extension updates does not find the updates to this extension; I speculate but don't know whether that is because this is an experimental extension.
Thanks very much for putting this together -- very convenient and useful.