XPCOM.dll is a library file used by the Firefox browser.
You can get additional information from here:
https://developer.mozilla.org/en-US/docs/XPCOM
You may need to reinstall Firefox. It's the easiest fix.
You may download the file by itself and place it manualy in the \Windows\System32 folder.
You can get it here:
http://www.dll-files.com/dllindex/dll-files.shtml?xpcom
Get the .ZIP version, extract to the correct location.
You don't need to get and install the fixer that you'll use only once.
It's possible that some Forefox addon that you are using are out of date and no longer compatible with the current version. Make sure that all addons are up to date. Make sure that Firefox is set to automaticaly get and install addons updates.
Disable or uninstall those that don't work and don't have any update available.
It's also possible that you are trying to load some programm that expect an older, obsolete, version of XPCOM.dll and use some hard coded, possibly undocumented, entry point. In this case, get the latest version of the offending programm, or try to find some alternative.
Electro, October 2013
