Just go and download the kitkat firmware from sammobile.com/firmware. Extract the zip file to get the tar file
Copy this tar file to any folder in your PC
Use the software ODIN ( google for it - suggest you use lower end version 1.87) and launch it
Swift off phone. Press vol down+power simultaneously and you will see download mode ( select vol up if prompted). Connect phone to PC
Launch odin and just select the tar file as PDA. Leave everything else as it is. Press start and the firmware will flash over in about a minute
Your data will remain after flashing

Strange you got such issues for lollipop - you are one of the rare users reporting such. You can try go to settings/app and restore everything back to default ( at bottom of screen) and see how it goes
If you had installed some unofficial apps ( ie pirated, apk from unknown sources rtc) - these will at times cause funny issues. Uninstalled such apps if you had installed and monitor

i don't get this scenario,  neither did most of us here,  try clearing cache, if can't then do a clean install again

You suppose to hard reset after upgrade.  U see many apps dont work or crash.

Warranty not affected as you are flashing original firmware

Its due to lolipop even my samsung app to print wireless dont work after upgrade. Before it works fine.

I recommend u do that but please dont restore the old app but install 1 by 1

Try reinstall facebook. Before that clear cache clear data.

If fails then report it to developer.

Definitely app prob.

Then like sifu benny said.

Ur app crash with other apps. Safest way is... fac reset

Yup. Its pretty much reformat ur phone and restart like when u unbox ur phone.