Its unfortunate that using Firefox is a decent identifying feature by itself these days. Hoping for a change in that trend.
Yes, agents change relatively rapidly, so unless trackers have a model predicting User Agent change, a tracking profile is only good for only so long.
Still, I suppose that one could either try to find the most common User Agent string, and set your browser to that, or have it change somewhat stochastically (but only enough so we don't compromise legitimate uses)