Note: From the comments, it appears this is not a feature-request, but a bug isolated to Firefox.
The confirm popup that saves you from losing your work when leaving the edit page...
You have started writing or editing a post.
Press OK to continue, or Cancel to stay on the current page.
... seems to be pretty smart about not coming up when you didn't really edit anything. But, in Firefox there is one small problem.
It seems to use some type of keypress detection to trigger the "modified" state. I frequently open the edit window to copy some raw text and paste it somewhere else. Other browsers only detect actually entry keys, but I'm getting the popup after simply pressing Ctrl+A or Ctrl+C
Then, when I close the window or click back, I have to dismiss the popup before continuing. It's not a huge inconvenience, but thanks to some genius in the browser cabal, I can never get used to "OK" being the same as "throw away my work", so it takes a split second of thought.
Browser: Firefox 3.6, Mac OS X