Do you have a home page or some other page open in the browser which is generating this message? If you can't fix the problem, create a new profile with Firefox closed.

https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles#w_start-the-profile-manager-when-firefox-is-closed

I don't have any pages open in the browser, because my Firefox browser won't open and won't start. When I try to open it, I get that error message I described in a window "XUL Runner". When I try to follow the instructions you sent, Firefox still will not open or start, and I get the same error message. When I try to open the profile page from the search box in the Windows start menu, neither Firefox or the profile page will not open or start, and I get the same error message. The way I was able to access my e-mail to send and receive messages was to install the Google Chrome Browser, which I am using now. I would still rather use the Firefox browser, especially since it has stored all my bookmarks.

An error message about XPCOM missing or about platform version usually happens because of a failed Firefox update that left you with a mixture of old and updated files in the Firefox program folder. This can happen in case security software was keeping a handle of some of the files in the Firefox program folder preventing Firefox from replacing all the files. You need to do a clean reinstall and remove the Firefox program folder to ensure that all files are replaced. You may have to disable security software temporarily in case the problem persists.

• https://www.mozilla.org/en-US/firefox/all/#product-desktop-release