I am facing the same problem in windows 10. see https://support.mozilla.org/en-US/questions/1140455

I have reverted back to 48.0.1 for now and have disabled auto update

Will Firefox launch in its Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

First, make sure firefox.exe does not appear in the Task Manager (Ctrl+Shift+Esc) -- on Windows 7 check the Processes tab, on Windows 10, it might be on the Details tab. You can select firefox.exe and End Process there.

Then:

Hold down the Shift key when starting Firefox.

A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

WTC71 said

FF 49.0.1, Window 7, 32-bit, Dell Latitude laptop. Firefox stopped launching, i.e. opening the UI. Fails to appear in the Task Manager "Applications" tab. Does appear twice in the Task Manager "Processes" Tab. Two sizes 15576K and 3700K. I tried uninstalling, rebooting, and re-installing. The uninstall appeared to run successfully. The subsequent installation appeared to run correctly. All file folder have correct date/time stamps. Perhaps there some residual file that needs to be removed in order for Firefox to initialize properly? If so, what/where is it? HELP, please.

Reverting to the previous version solved the problem. 49.0.1 doesn't work on the 32-bit laptop.

Thanks!