Wouldn't that be easy to verify by just looking at the relative market shares of different browsers and checking if there is an 'unknown' browser whose market share is increasing proportionately to Firefox's decrease?
Not if the metrics come solely from analytics like GA. It would appear as an absolute decline in browser numbers instead (which would probably be indistinguishable from noise in most samples).
No, because there are multiple independent variables there: OEM mobile browsers, for example, and the fact that the overall number and distribution of observed browsers is not static.
