Great expansion! I don't have to add "/archive" in URL's manually for blogs, where "Archive" button is disabled. This is what I was looking for. Thanks, Alex!

But you need to do something with blogs who have custom URL's. Not every blog have ".tumlr.com" at the end

It would be good if the extension remembered the options, and wasn't randomly affecting all tumblr tabs. (why would someone want to turn all tumblr tabs to an archive anyway?: with many tabs, it will affect firefox & CPU perfs a lot).
Also, it shouldn't affect the dashboard or images/media opened, whatever the settings are.
This extension is a good idea, but needs a serious polishing.
Also, could we get the link to the source code?
Keep on improving, developper, this could be a cool extension. I'll change my review accordingly ;)