Actually, I take that back because the game won't run at all under my main Firefox instance. It seems to run fine on a second Firefox instance (using a separate shortcut with the command line "C:\Program Files\Mozilla Firefox\firefox.exe" -p "dd" -no-remote ) which is what I was using to play it when I made my last post, but it doesn't even get to the log-in screen with just the normal Firefox process. It gets " the content was stopped because a fatal error occurred" or whatever that error is.
edit: closing down and restarting my primary Firefox instance made the game run fine, at least for awhile (I only gave it about 20 minutes). Apparently it doesn't like to run if Firefox has been open for a long time, and mine runs for days sometimes weeks without being closed (although its system usage doesn't change or anything).