I use NetSetMan Pro to switch between customers network profile and it started to complain about the size of my FF profile (141,7MB).
I opened the folder and found thar prefs.js was more than 140MB and when I opened it I found that there is something wrong with "services.sync.client.name" key.
I think that alue should be something like "Puccio's Firefox on SC-GUGLIELMIA" but it is "PuccioÃƒÆ’Ã†â€™[CUT more than 120MB]ÃƒÆ’Ã†â€™s Firefox on SC-GUGLIELMIA".
I will make a copy of the original prefs.js and try to fix the key value but from outside FF as trying to edit from about:config locks it.
Mozilla/5.0 (Windows NT 10.0; WOW64; rv:51.0) Gecko/20100101 Firefox/51.0 ID:20170125094131 CSet: 327e081221b064b05a302d7877c6e4be2949a617