It's being outpaced by other browsers in a rapidly expanding field, not falling in usage.
Safari and Edge are defaults on iOS, MacOS, and Windows, and Chrome is the default on Android. I imagine we're primarily seeing the effects platform capture everywhere else.
Firefox says they have 200m monthly active users. In 2018, techcrunch wrote about that same report and said "this data shows a downward trend for monthly active Firefox users, which now measure about 250 million, down from well over 300 million last April."
So I would be concerned.. they're actively bleeding users... not just being outpaced in a growing market.
Safari and Edge are defaults on iOS, MacOS, and Windows, and Chrome is the default on Android. I imagine we're primarily seeing the effects platform capture everywhere else.