Firefox 4 Back Button bug with prototype and zepto Adapter

When using the prototypeJS or zepto Adapter, the back button in Firefox 4 doesn't trigger "statechange". It works with jquery and mootools. 

The problem is the line "event.state = (event.state||false);" at which FF stops. I moved the line into the "if ( typeof event.state === 'undefined')" condition and it works.