Hi there. As you pointed out yourself this is not really a limitation of the extension as such but in how Firefox handles files loaded directly. This extension was written specifically to handle the case when a TIFF file was included in a page, and hence cause the web browser to make a request for such a file. When you load a file locally it doesn't cause such a request and the extension can not pick it up.

I will look into adding the ability to pick up direct TIFF file opens, it should be possible, so look for that in a future version :)

EDIT: I did look into it and extensions can not register file handlers, only applications can do that and applications can not do some of the other things that this extension needs, like intercept page requests. So this is a no go unfortunately.