The FB has shapers but they aren't really the same thing as proper QoS. Proper QoS can also be CPU intensive however. The way the FB does it, as far I'm aware, is like AAISP's website describes:
"We have options allowing users to set their line at a lower rate, e.g. 95% of BRAS. This is useful for things like VoIP as our shaper is smarter than BTs and allows small packets through more readily than large packets. Customers have tested VoIP on a 95% rate whilst running torrents and downloads and found it works well."
So while you can adjust the rate limit of the downstream from their control panel, you must rate limit the upstream from your side with an appropriate router such as the FB.
The closest thing the FB has, that I'm aware of, is when you have bonded lines you specify the upstream bandwidth of each line. I've set mine about a megabit lower than the actual average upstream sync rate (e.g. 11 megabits instead of 12 megabits). I don't have such issues with my two connections, on dslreports speed test with down and up having six streams I get a bufferbloat grade of A and a quality of A+.
However, for a much slower DSL connection then things could be a lot different and you might find you'll need a router between the FireBrick and your LAN devices which does fq_codel. A cheap EdgeRouter might be an option to consider, I used fq_codel on that (Smart Queue as they call it) when I was using my EdgeRouter.