Depending on your speeds it may not be enough to keep up with what the firestick needs to stream clear video. To fix this try the following steps to see if it resolves the issue.
Step 1: Launch the Fire TV Stick menu and select Preferences. Step 2: Select Data Monitoring. Step 3: Tap Data Monitoring again to reveal the Data Monitoring options. Step 4: Select 'Set Video Quality'. Step 5: Try the "Good" setting to see if it fixes the video.