I've gotten this a few times today, in an m-c from yesterday. STR: start the browser, open a page, open about:memory, hit minimize memory usage. I'll attach the stack, but it looks like on the main thread we're dispatching the memory pressure thing from the observer service, which ends up in mozilla::storage::Service::minimizeMemory(), which then does a dispatch, which ends up in mozilla::LazyIdleThread::OnDispatchedEvent, and then we hit this assert.
Created attachment 8859356 [details] Other storage stack I also see this stack at startup sometimes, in this profile, so maybe the profile is just messed up? This is a default profile generated by mach run.
It looks like the same as bug 1239794