Paste any unicode flag into anywhere on the Forum website; in the search bar, a post, user profile... and instead of the flag graphic, the alphabetic code is shown. It used to display the graphic flags until recently.
For instance, paste this: "🇬🇧"
That should show a graphic British flag; it does here, but on the SL forum website on Firefox it shows the letters GB instead.
While partly a Windows/Browser issue, the flags work on any other website in Firefox. Just not SL's forum. Note that the flags also seem to work on the Marketplace website.
Clearly something changed recently to stop them working; please make them work again.