it's now more than a year, that this XUL error (in mozilla and firefox) infected all my crossbrowser codes when i try to set the focus() to an dynamic generated <input> field.
It seem to be a internal bug, even in firefox 1.0. (see bugzilla).
There were many tips from setTimeout() to just ingnore the error.
making the spreadsheet editor for the grid, i found somewere deep in bugzilla the solution:
<inputautocomplete="OFF"type...> var iX = document.createElement("INPUT");
i don't know what this exatcly does, but with this attribute, you can focus() or select() all input field dynamically without NS_ERROR_XPC_JS_THREW_STRING.
see here the spreadsheet-grid: http://webmail.mbn.ch/table/dyntable.php