RJSdisplayed when using back button?

Given an action that responds to regular requests and XHR requests.

if request.xhr? then
  render :update do |page| page.visual... end
elsif !request.xhr? then
  render :template => ...

Using the browser back button to return to this page, firefox 2 will
'sometimes' display the raw javascript of the last XHR response and not
the base page.
Oddly, it doesn't happen if going back one page, but happens when going
back several pages.

Maybe a browser issue, but wondering if anyone has any suggestions?

Many Thanks
Andy Watts