Yes, a slow memory leak can be a hard thing to pin down for this reason. Can you expose the problem by forcing the UI to be more hyperactive? For example, code a short Time.every subscription to perform some action lots of times very quickly? That could be making a call to fetch data from the back-end or just making some change to the Model that will re-render the UI slightly differently each time and so on.
I am also thinking, so Firefox allocates a lot of memory but does not release it easily. Is this actually a memory leak? or is it just how Firefox likes to work? That is, if there is plenty system memory available, perhaps it likes to avoid compacting the heap, so just hangs onto memory - so long as plenty is available. Can you force its process, or the OS to have less memory available, and see if it behaves more conservatively under those restrictions? What I am saying is, is this really a leak or just typical firefox behaviour that is to be expected?