URL: https://m.youtube.com/watch?v=wIft-t-MQuE

Browser / Version: Firefox Mobile 61.0
Operating System: Android 6.0
Tested Another Browser: No

Problem type: Something else
Description: When the video is in fullscreen the subtitles jump around
Steps to Reproduce:
When you are playing this video in fullscreen with the subtitles on, the subtitle jumps around the video drastically. Sometimes half the subtitle is cut off as it is on the screen edge.

From webcompat.com with ❤️