See "gmail is slow or stops responding": https://bugzilla.mozilla.org/show_bug.cgi?id=1380896
Which is actually a duplicate of "Youtube Live video stops after about 5-10 minutes": https://bugzilla.mozilla.org/show_bug.cgi?id=1381016
Which is fixed for:
- Firefox 52 (ESR)
- Firefox 55 and above.
It's marked "wonfix" for Firefox 54.