It doesn't seem to understand what .xpi files are. Now, xpi files are just zipfiles, and it's possible to uncompress them using "unzip". The next problem is what to do with the files. Looking at a PC version of Firefox they seem to go into the "extensions" directory under "browsersync@google.com", but after that I have no idea. I suspect the settings in prefs.js get added by the installer too when it goes through the first-run wizard.