Firefox_40198241_pl.mkv Review

(e.g., watch it, convert it, or find out who sent it?) MKV Files Explained - Adobe

While Firefox has recently added better support for MKV playback, you may still need to use a dedicated player if the video stream inside is encoded with a specific codec like HEVC. Firefox_40198241_PL.mkv

(e.g., your Downloads folder, a specific website, or a program directory?) your Downloads folder

The filename appears to be a specific video file, likely a screen recording, a downloaded clip, or a technical log from the Firefox browser . a specific website