I have added the continuous scroll mode. It's available in the 0.3.0.0 version of the extension. You can try it out.
However, I didn't understand what do you mean by adding a context item for local directories opened in Firefox (I don't use Firefox actively, mostly Opera). You can send me a screenshot of such directory, with explanation where do you want to see a context menu, at email@example.com
And for the dark background I have taken it into account, and perhaps will add it in the future versions.
You are welcome! When I created the extension I kept in mind that even if it can't be a substitute of desktop applications, it still may be used by those, who can't install a program, but need to view a djvu file. Thus, I'm glad the extension helped you :)