Thanks for your response.
Yes, all plugins are off.
I just read what the problem was.
(https://stackoverflow.com/questions/4263536/firefox-cache-textarea-value)
I didn’t realize it has nothing with WordPress or TinyMCE, but is the default behavior of a textarea in Firefox, and I need to set an attribute I never heard of, autocomplete.
<textarea autocomplete=”off”></textarea>
So for TinyMCE I added the following code to the plugin I’m working on:
jQuery(‘#postdivrich #editorcontainer #content’).attr(‘autocomplete’, ‘off’);
I’m not sure if I needed to specify all of those element IDs to get to the text area, but this works. Hooray!