[xubuntu] Troubleshooting Firefox in 16.04

Hello,

After a recent reinstallation, I'm unable to open Firefox normally--I receive the error message,

Firefox is already ruing, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

I've tried completely removing and reinstalling FF, ruing killall firefox and trying again; I've deleted .mozilla/.firefox, then uninstalled/reinstalled and probably tried a few other things that are now crowded out of my gourd.

This 'feels' to me like a profile problem, but if it is I don't understand how to fix it.

I can start Firefox with

sudo firefox

Any suggestions would be greatly appreciated.

Thanks in advance.