When you insert a text, briefly you'll see a Paste icon next to the end of the text. If you move the mouse over it, a downwards arrow appears, with whoch you can open a pull-down menu. There you can chane it to plain text.

However, this should not really bother you, as if you re-insert your draft from Word into the ET edit window in Firefox, the Word formatting will disappear anyway.

_{*Lunatic*, n.
One whose delusions are out of fashion.}