What was most questionable was it's location (C:/prefs.js). Nothing was seemingly broken or changed within windows or the browser after deletion, and only one antimalware program I tested (gridinsoft) reported it as malware 'Trojan.PL.Gen.bot'. I scanned with bitdefender,avast, malwarebytes, no result. virustotal gave me 2 results - I still have the original file that I will try and upload later if necessary, but have re-installed windows since then (to give me back some hdd space - as I'm on an older laptop). I had not installed anything new before noticing it.
Thanks for any info or opinion- I know the browser uses prefs.js but it is normally located in the program's installation and appdata folders, and has far more regarding content. All I can think of is that the above may have been the creation of a possible plugin (I had ublock,acrobat and java deployment toolkit as an extension, but that is all).