This happened multiple times to me too. I'm using Firefox 3.6.10 on Mac. This happened twice after I accidentally detached a tab window that I was in the middle of writing out from the main browser window. The cursor disappeared and I couldn't do anything. It came back after I refresh the page. (I had to copy whatever I was writing before I refresh the browser not to lose the content)
But as cheflayne suggested, the cursor is back after I switch to source mode by clicking the Source button in the toolbar and then click the Source again to come back to the wysiwyg editor.
I use Firefox exclusively in Linux and Windows and haven't had the problem though this cursor issue is a known bug in how this platform interacts with Firefox. If you've upgraded to the latest version of Firefox (3.6.11 (this week I think) most of this should be gone.
Future versions of the platform are planned to have a simpler more compliant editor option.