In my case Memrise can take 25% of CPU (one core) which makes Firefox sounds distorted. Generally it is huge scandal that web site can eat so much CPU power. What kind of programmers have you there?
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quoting myself above
When I looked at it last week, the conclusion I came to is that it’s caused by the atrocious code that Memrise is using on the web site, rather than being a particular bug that can be solved. Their code is the kind of thing that could cause permanent trauma and recurring nightmares in those who gaze upon it.
I just did a trace of memory allocations after a 3 level reviewing session of about 15 items total reviewed. It allocated over 25,000 objects, and used over 32MB of memory for those few items. 
I agree with you on FF Quantum. I don’t think they’ve gotten all the bugs worked out yet, and I’m seeing a lot of user reports of it breaking different web pages.
But in today’s test that I just did, I used chromium and its memory tracing tools, and those were my results. And they’re consistent with the results in the different versions of Firefox, both quantum and pre-quantum ESR version. All of them are showing very high and ever-climbing memory usage on Memrise for me.
Not that I’ve noticed, but I only do typing and multiple choice, and have the listening tests turned off. I’m not a pro user, so I don’t have videos or anything like that running either. I imagine that users who have those multimedia tests turned on would have even worse resource usage.
That’s why it’s so shocking to me to see so many objects and so much memory allocated for just 15 typing test items. There’s no rational reason for it to be so bad.
Today I did 1 review session on the web, 25 words (PRO user, but no video or audio in this part).
Did before starting the session (but after opening the course, just before pressing the button), a ‘Heap’ snapshot: 25 MB. Did one after the session: 85 MB. That’s is 3 times more, but not the 500 MB you were talking about. Of course, I did not only one review session.