back button sometimes goes to previous URL but does not reload page
I have a number of friends lists in facebook. I bring up the list of friends lists. Click on one to go to it. Then I click the FF back button to go back to the list of friends lists. Sometimes that works and sometimes recently it display the URL for the list of friends lists in the address bar but does NOT reload the page so that I am still looking at the content of a friends list rather than at the list of them.
So what would make the FF back button go back to the previous URL (in the address bar) but not load that page?
I have a number of friends lists in facebook. I bring up the list of friends lists. Click on one to go to it. Then I click the FF back button to go back to the list of friends lists. Sometimes that works and sometimes recently it display the URL for the list of friends lists in the address bar but does NOT reload the page so that I am still looking at the content of a friends list rather than at the list of them.
So what would make the FF back button go back to the previous URL (in the address bar) but not load that page?