I had a similar event with Firefox 43.0.1, EdgeOS 1.8, ERLite-3 the other day.
Same symptoms, router reset itself. Cleared browser cache, restarted it and back to normal. I don't think the modem reset but cannot confirm; there is no entry in its log. From IE 8.0, it was still working, until the reset.
I just purchased this router and was up and running for 3 weeks straight. It is a bit disconcerting that it has already had a serious issue while my previous Linksys EA6400 didn't in over a year!
There are issues with the GUI - more inconveniences than issues, really. I can certainly live with those but a "professional" router resetting like this...not comforting.
-Arthur