Some web pages are 'written in stone' meaning they never change. Others, like news and other such pages, are updated a lot. So the page you started from may no longer exist.

I get around this by opening the link I want in a new tab/window. Once I am done on that page, I close it. Ending up where I started.

You can get a "document has expired" message if you use the back button or a bookmark to go back to a specific page and the web site has instructed Firefox not to cache the page. Firefox tries to retrieve the page from the server and possibly sends POST data that is no longer valid. In such a case best is to open links from such a page in a new tab/window by middle-clicking a link to avoid using the Back button, but instead close the tab to return to the previous tab.