I'm running 64-bit Windows 10 (Ver. 1607, build 14393-693) on a Toshiba Portege R835 (8-GB RAM). I've had no previous problems with any Firefox updates and have made no hardware or software changes (adding, deleting, etc.) - outside of whatever automatic updates Microsoft has forced onto my system. Beginning a week ago (+/-), the familiar "Update Firefox" dialog box has appeared four times. Except for the first occurrence (during which I was involved in a project), I selected 'Update' - each of the next three successive appearances of the same dialog box (all three of which were prompting the Ver. 51.0.1 upgrade). On each of those three occasions, I selected "Update, after which the progress bar completed its left-to-right fill normally and the usual "Restart Firefox" prompt appeared (with no error message or any other indication suggesting an unsuccessful update). Selecting "Restart," did eliminate the 'restart' prompt (ergo shutting down Firefox?), but Firefox did not restart as usual - there was, again, no error message or other indication of an unsuccessful update. Attempting a manual start at that point still did not open an active Firefox window, however, when Task Manager was opened, it showed two 'running' instances of Firefox within the list of active processes. After ending both active processes via Task Manager and starting Firefox manually (again), an active Firefox window did appear. Sometime later, however, the same 'Update Firefox' dialog box appeared (again prompting upgrading - to Ver. 51.0.1). This has now occurred on three separate occasions - each one yielding the described results. The same 'Update Firefox' dialog box (to Ver 51.0.1 yet again) appeared once again (the fifth time thus far) a short time ago (~5:00p.m. est; 30 January 2017). I have searched your Web site - as well as the Web - for any reports of the same/similar problems, with neither source proving helpful - ergo I am submitting this question. Should you need/want additional information, please advise. Thanks.