--- a/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp
+++ b/dom/media/platforms/ffmpeg/FFmpegVideoDecoder.cpp
@@ -140,17 +140,18 @@ FFmpegVideoDecoder<LIBAV_VER>::InitCodec
   if (mInfo.mDisplay.width >= 2048) {
     decode_threads = 8;
   } else if (mInfo.mDisplay.width >= 1024) {
     decode_threads = 4;
   } else if (mInfo.mDisplay.width >= 320) {
     decode_threads = 2;
   }
 
-  decode_threads = std::min(decode_threads, PR_GetNumberOfProcessors());
+  decode_threads = std::min(decode_threads, PR_GetNumberOfProcessors() - 1);
+  decode_threads = std::max(decode_threads, 1);
   mCodecContext->thread_count = decode_threads;
   if (decode_threads > 1) {
     mCodecContext->thread_type = FF_THREAD_SLICE | FF_THREAD_FRAME;
   }
 
   // FFmpeg will call back to this to negotiate a video pixel format.
   mCodecContext->get_format = ChoosePixelFormat;