I understand that when you add a new device to the sync account. Try to sync it from another menu:

1. Go to accounts.firefox.com and sign on here
2. From the Android menu there is a list of accounts, sign out from Firefox sync listed in this menu

From which one is it crashing? Are there any crash reports generated? about:crashes

hi

ive tried the above and tried all possible ways of adding an android sync account and it still crashes. I have given up i guess it must be some issue with xperia devices not liking firefox sync as it seems to be quite common. ive gone back to chrome until this can be fixed.

I have same trouble on same device. when sync runs, Firefox crash every time.

its one of my crash report: https://crash-stats.mozilla.com/report/index/d97baad6-d357-4065-95ad-b01ea2151003

another person mention this on reddit: https://www.reddit.com/r/firefox/comments/3mqf17/android_firefox_sync_always_crashes_when_logging/

Hi Tonbi_ko, I have mentioned the bug and the thread to the sync team and will try to find more information, please keep an eye out for a follow up in the bug report mentioned in reddit.