This is a bug associated with firefox. From this source: https://bugzilla.mozilla.org/show_bug.cgi?id=1215012.
It is also suspected to be caused by the patch Support Multicast DNS (mDNS)
Further tests showed these results:
Excessive battery usage when using Firefox beta.
Phone Power setting shows MSNSD with increasing usage.
This issue was mainly noted in beta version of firefox and version 42.
Several users have complained about the issue and this how they resolved it:
Install this new version, run it once, and the mdnsd process starts
busily running your battery life down. It took my S5 from 17 or 20
hours before the battery would run out to 4 hours. This is true
whether or not Firefox is running in the background. All the
complaints on the web about this start around November 2014 time frame
with complaints about the two other random apps, but only a couple,
and a comment on the Mozilla Bugzilla platform that there was a report
of extreme battery usage sometime in July of 2015
Source: What is MDNSD and how do I get rid of it?
Uninstalling firefox proved to be the solution:
So by uninstalling Firefox, the mdnsd process stopped draining the
battery for the rest of the day. After recharging the battery
overnight and beginning to use the phone the following day, the
Battery usage report no longer shows the mdnsd process!
Source: Mdnsd killing battery?
Probably the issue was subsequently fixed with newer versions of the browser.