URL: https://www.twitch.tv

Browser / Version: Firefox 57.0
Operating System: Windows 10
Tested Another Browser: Yes

Problem type: Something else
Description: Extreme memory leaks while watching a busy stream
Steps to Reproduce:
While watching a busy stream on Twitch.tv, I noticed that Firefox starts to use a lot of memory after a few minutes, ranging from 200MB (initially) to even 600MB~650MB tops.

This doesn't happen on Chrome, where the memory usage ranges between 250MB350MB tops. The CPU usage is also higher in comparison (1013% on Firefox vs 5%~10% on Chrome).

Steps to reproduce:

1. Open Twitch.tv (https://www.twitch.tv)
2. Go to a famous stream with a lot of viewers (a busy chat seems to exacerbate this problem)
3. Watch it for some minutes while also interacting with the page a little bit, like going from top to bottom to spawn the "mini video po-up" or going in and out of theather mode

Is this a memory leak problem? I didn't find any bug on bugzilla related to this issue.

Note: this happens both on v55 (stable) and v57 (beta).

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