4 beta: ConfigParser.NoOptionError: No option 'lastversion' in section: 'Compatibility'

Binary package hint: firefox

Firefox 4.0 Beta refuses to start since two days ago:

Traceback (most recent call last):
  File "/usr/lib/firefox-4.0b8pre/xulapp-profilemigrator", line 201, in check_candidates
    profile = ProfileFolder (os.path.join (os.path.dirname (this.profile.path), f))
  File "/usr/lib/firefox-4.0b8pre/xulapp-profilemigrator", line 152, in __init__
    this.last_version_string = get_last_version_from_profile (this.path)
  File "/usr/lib/firefox-4.0b8pre/xulapp-profilemigrator", line 48, in get_last_version_from_profile
    return get_last_version_from_compat_ini (subprofile)
  File "/usr/lib/firefox-4.0b8pre/xulapp-profilemigrator", line 77, in get_last_version_from_compat_ini
    return re.sub (r'_.*', '', compat_ini.get ('Compatibility', 'LastVersion'))
  File "/usr/lib/python2.6/ConfigParser.py", line 541, in get
    raise NoOptionError(option, section)
ConfigParser.NoOptionError: No option 'lastversion' in section: 'Compatibility'
^CTraceback (most recent call last):
  File "/usr/lib/firefox-4.0b8pre/xulapp-profilemigrator", line 420, in <module>
    if (migrator.main () == True or not os.path.isfile (options.profile + '.series-stamp')):
  File "/usr/lib/firefox-4.0b8pre/xulapp-profilemigrator", line 349, in main
    gtk.main ()
KeyboardInterrupt