I had the same experience (lost ~15 minutes of writing) when I wrote my long message above (8:55, 2 December 2009). I was patrolling vandalism at the same time and accidentally clicked and opened a link in the same window. I went back to this page but my text was gone, which normally doesn't happen in Firefox.
Yeah, Andrew told me this morning that he's working on EXACTLY that issue - making sure that the text is buffered or something so that it's there when you hit the "back" button.
same issue. working on a Mini 10 with small keyboard and literally grazed the mouse pad, the page went forward and when I went back I had lost everything. At one point I had copied it to clipboard but I had just copied a URL from another window to share and that knocked my buffer for the clipboard. :( Will rewrite in a bit. Busy day.