Do you anticipate the losses are due to people with aging machines upgrading and using a different browser? As a very happy Firefox user, I have a very hard time believing it's because people find the experience to be poor. I feel as if the losses you are describing have to be losses to defaults on new devices and people that fairly don't care enough to change them.

The desktop is virtually dead for any "normal" user, and their Android or Apple tablet/phone don't prompt them with a choice (other vendor constraints notwithstanding). It's not remotely a fair fight. You are guaranteed to lose users for reasons totally outside your control.