Yup. Firefox's Isolated Web Container process just chomps up all the memory of your system. It's either a memory leak in Firefox or a memory leak in Lemmy. If you downgrade Firefo it seems better, but Lemmy seems to take quite a bit of time to load. At least in my brief testing.
Sadly I don't know how to custom build Firefox, nor have the time, to run it through Valgrind to find the leak. I tried with my standard firefox and it just never launched.
I'd recommend posting to the Github to see if Lemmy devs pay attention.
EDIT: I actually posted this earlier: https://beehaw.org/post/678781