I just today had the same problem (.htmlText not working in firefox), and found this page when I googled for help (sorry am no customer).
Thanks for XMLSerializer-info. this works great!!

In the following i am doing the same as the OP does with his "editor1.PasteHTML(selectedhtml + Date());" and run into the next problem which the OP may als get:
when the end of the selection contains different formated text (i.e. bold) than at the beginning of the selection, in the variable "selectedhtml" this formated text gets closed (and reopended again just after the end of the selection).
when using "selectedhtml + Date()" to add the Date, that text will be inserted just in between the closing and the opening-tags. so date wont be bold even when it should be.
how is this solved?