Yes, there was a security patch which restricts local file loads by local pages. As noted in the solution at the top, you can roll back that patch:

(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 uniq and pause while the list is filtered

(3) Double-click the privacy.file_unique_origin preference to switch the value from true to false

Why is my XSL file no longer being applied to my XML file?