Do a clean reinstall and delete the Firefox program folder before (re)installing a fresh copy of the current Firefox release.

Download a fresh Firefox copy and save the file to the desktop.

• Firefox 25: http://www.mozilla.org/en-US/firefox/all.html

Uninstall your current Firefox version, if possible, to cleanup the Windows registry and settings in security software.

• Do NOT remove personal data when you uninstall your current Firefox version, because all profile folders will be removed and you lose personal data like bookmarks and passwords from profiles of other Firefox versions.

Remove the Firefox program folder before installing that newly downloaded copy of the Firefox installer.

• (32 bit Windows) "C:\Program Files\Mozilla Firefox\"
• (64 bit Windows) "C:\Program Files (x86)\Mozilla Firefox\"

• It is important to delete the Firefox program folder to remove all the files and make sure that there are no problems with files that were leftover after uninstalling.
• http://kb.mozillazine.org/Uninstalling_Firefox

Your bookmarks and other personal data are stored in the Firefox profile folder and won't be affected by an uninstall and (re)install, but make sure that "remove personal data" is NOT selected when you uninstall Firefox.

If you keep having problems then also create a new profile.

• http://kb.mozillazine.org/Profile_folder_-_Firefox
• http://kb.mozillazine.org/Profile_backup

• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Clean_reinstall

Thanks cor-el but your reply is a copy-paste of a similar question on this forum, which I've already read and tried.

As I wrote above, I've already uninstalled firefox, I've deleted both program files and personal data folder (I don't care about profile data) inside users\user\appdata etc.. I used also ccleaner after that. Then I reboot windows and installed a new copy of firefox, just downloaded. It didn't help.

what kind of security programs are running on your computer?

also see http://kb.mozillazine.org/Browser_will_not_start_up#.22Couldn.27t_load_XPCOM.22_error

AVG free and win defender as antivirus and comodo as firewall. I will try this evening what is suggested in your link, thank you. Anyway I've already tried to disable the antivirus with no success.

This is usually a problem with security software that blocks files in the Firefox program folder.
You can try to boot the computer in Safe Mode and do a clean install in that startup mode.

cor-el's post is right on!

I have Webroot Secure Anywhere AV. Recently, it's giving me quite a few headaches where my previous Avira FREE never did.

The problem was in the Identity Protection.

To determine which component was causing the issue, initially, I disable all security software - Private Firewall and Webroot AV. Everything works fine.

I then enable Private Firewall first and all worked fine.

Next I enabled 1st part of Webroot AV (PC Security). All worked fine. I now know it's the Identity Protection that's causing the problem.

Lastly, I locate the components that's are blocked - they are: Firefox.exe. In my case, it's Palemoon.exe, but it's applicable to Firefox as well. There are two other DLLs that's being recorded: nss3.dll & xul.dll.

Once I added all 3 files to the allow list, everything works great now.

I had the same problem "firefox couldn't load xpcom". What I did is just uninstall it , and then reinstall it again , without removing firefox folder data. It was very simple for me :) thank god