This is very interesting. klik by design allows to run many versions in parallel. In this case, it seems, that the configs written by 1.5 mess up 1.x - this is certainly bad behavior on the side of Firefox.

If others can reporoduce this error, we should probably consider submitting a bug to the Firefox bugtracker.

Imho, it would be good behavior that a newer version of an app shouldn't render the settings of an older version unuseable... otherwise people can't use multiple versions at once (which klik enables you to do), and cannot downgrade.

Can anyone confirm the - let me call it - bug?

Greeings,
probono