Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. A small dialog should appear. Click Start In Safe Mode (not Refresh).

Is the problem still there?

Thank you for your reply, FredMcD, but unfortunately the fix does not work. Actually, last night I tried every remedy available on mozilla support forum, like the one you posted, but it didn't last night nor today trying it again. I've also re-installed Firefox several times as well as run the updater function in about:firefox, but nothing seems to faze it. No new add-ons, etc., were applicable to the possible problem, either. I just had been using Firefox as always during the evening and, from a random attempt to re-adjust position of the window, I discovered the mouse could not move the window. Nothing was done to make the browser unstable, etc., such as messing with add-ons or extension settings, nothing at all. Peculiar, though, is that other browsers work perfectly fine. I updated the other two I have to goose up security on them, add ad-block, etc., so that I could use them the way I like. But, I've been using Firefox like since it began, so this problem is annoying, especially since it has no reason to stop functioning like it has as far as moving a page.

Update on mouse function problem with Firefox:

Pondering the situation, I thought of scheduling a chkdsk c: /f and restarted. The problem seems to now be resolved. Since rebooting after chkdsk, the mouse function seems to work as usual as I've been closing & restarting Firefox and trying it with several web sites, etc., and all seems well. So, though the problem only affected Firefox, the chkdsk /f in this instance has worked for a fix to the problem. Maybe scheduling a chkdsk /f will help others experiencing this odd little problem.

That was very good work. Well done.

In the last few days, I too have had the problem that I "cannot reposition Firefox screen with mouse." The problem occurred suddenly, but perhaps tellingly just after I had "refreshed" my Firefox program. I found this page describing my problem. But I didn't know how to do the suggested "chkdsk c: /f" solution. A good thing, I suspect.

By pure chance I have apparently found how to move the screen (window?): Click and hold the L mouse key with the mouse pointer no more than 1 mm below the upper window border, then move mouse pointer and the window moves. Below that narrow band, the mouse does nothing.

Perform a System File Check to fix corrupted system files Win 7, 8, 10, Vista: https://support.microsoft.com/en-us/kb/929833 Win XP: https://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/system_file_checker.mspx?mfr=true

See if there are updates for your graphics drivers https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration