xpcom.dll error comes due to different reason,and if the reinstallation does not restore the dll file then run sfc/scannow in admin cmd prompt. It will verify the corrupt dll file. You can do in other way that try to install firefox into new directory (previous was Program Files/Mozilla Firefox) and new one is another directory. This will solve the problem.

xpcom.dll is no longer used by Firefox, so it it likely that you have outdated software or malware that tries to access this file.

You can check for recently installed suspicious or unknown extensions.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked
• https://support.mozilla.org/kb/troubleshoot-firefox-issues-caused-malware

Bug 852950 - Kill xpcom.dll/libxpcom.so/xpcom.dylib

I uninstalled and reinstalled Firefox to a different directory (however I was surprised to find it still installed in the program files as well as the new directory). This seems to have fixed the problems I was seeing in Firefox and Safari. However, I still cannot do a restore. I did the scan sfc/scannow as you suggested and it finds no problems. I don't know if the xpcom.dll file is still missing or not.

what error now you get when trying to restore ?

Microsoft. Net Framework Unhandled exception has occurred in your application.........................(Exception from HRESULT. 0X80040155)

I don't know how to add a file or screen shot here. It said "system restore did not complete successfully........... system restore could not access a file......................... an unexpected error occurred during system restore. (0x80070005). have tried multiple restore points.