Hello,
Could I ask which input file and codecs you're trying to use? We're not aware with any playback issues in Firefox.
The most commonly accepted codecs would be H264 for video and AAC for audio. If you're unsure how to check the codecs you can do it easily with 2 methods:
Using VLC
Open the file in VLC and press the "tools" menu button and select "codec information" or try pressing "ctrl+J" it should get you a window with codec information.
Using ffprobe
If you're using ffmpeg/ffprobe you can use "ffprobe file" to view the codec information.