This seems to be a common error some people encounter with Firefox. First, XPCOM has nothing to do with Windows XP, rather it is Cross Platform Component Object Model. It is a cross-platform component model from Mozilla (more info at Wikipedia). Not sure what causes this error other than a file or files for XPCOM in the main Firefox uninstall some how get removed or corrupted. However, the fix is fairly simple and painless: re-install Firefox.
Uninstalling Firefox will NOT remove your profile folder or settings, it simply removes the Firefox browser from your computer. Once you have removed Firefox, (using another browser) go to getfirefox.com to download the latest version of Firefox. Once you complete the installation, Firefox should launch without issue and with your profile.