I assume it's a recent version of the PowerPoint desktop application (not the website).

On the ribbon, if you click the triangle below the Paste button and choose Paste Special, does PowerPoint indicate that it has either "Device Independent Bitmap" or "Bitmap" available as a format to paste? If so, do either of those work?

I was using for paste keyboard (Ctrl+V). It was working normally until last FireFox update. It is also working properly from Edge and Chrome. When I do what you suggested it indicated 4 possibilities. But I found out that when I open right click menu in PowerPoint it offers all 3 standard paste options and 3rd is working ("Picture"). But I prefer keyboard.

Thank you for testing that. Which of the various formats in Paste Special has the problem? I know Word defaults to HTML, but I don't know about PowerPoint. Is the problem with the HTML format or one of the others?

"Use destination theme" and "Keep Source Formating". But with png files the third option (Picture) pastes only black rectangle. :D From the ribbon/Paste Special: HTML works well, FILES brings error message and both bitmap options paste black rectangle.

FILES gives the same error message as a plain paste? I don't know whether there is a way to change the default to HTML. The options panels have been simplified recently so I can't figure out where that would be.

Regarding black rectangles for PNGs, some applications will paste a black background in transparent areas of PNGs. This seems to be due to some difference in how Firefox stores them on the system clipboard.

Yes, FILES gives same error message as plain paste. Thank you for your explanation. But I still do not have solution. Everything was working fine until that update. So this is some bug or something was changed without taking this effect into consideration. Maybe someone in Mozzila or Microsoft would now. :)