Unfortunately that's not a likely explanation. If you do even the simplest possible measurement, which is just recording user agent strings and IPs, you won't end up undercounting Firefox clients
I was clearly wrong, though it is easy to change your user agent it's unlikely that many Firefox users have done it.
I'd add there is a large gap in usage between uBlock and Matrix. uBlock has over 10 million users whereas Matrix is somewhere over 100,000.