This is what is called protected web page and unless it is opened from their site it won't work. There is nothing FF can do about Web protection they implement to prevent saving their page.

Does the same issue occur if you open the page in Internet Explorer or another browser on your system?

What you're describing can be caused by a script in the page. As a one-time solution, you could disable JavaScript,* but that wouldn't be very practical if you have to do it every time you open a saved page from that site.

You could consider capturing a screenshot of the page or converting it to PDF in the future to work around this issue.

* To disable JavaScript globally:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk.

(2) In the search box above the list, type or paste javas and pause while the list is filtered

(3) Double-click the javascript.enabled preference to switch the value from true to false

I suggest keeping the tab open so you can switch it back once you're done with the problem pages. Double-click to switch the value from false back to true.