I ported this [chrome extension](https://chrome.google.com/webstore/detail/video-search-for-youtube/habdalaejnjehgaadeignhbmllkdngpf?hl=de) to Firefox. It lets you search the subtitles of a video, so you can jump to the part of the video that interests you.
Here's the [link](https://addons.mozilla.org/en-US/firefox/addon/search-in-subtitles-on-youtube/).