hi RichardNil, probably a clean reinstall will help in this case:

1. download the full firefox setup file from https://www.mozilla.org/firefox/all/
2. uninstall firefox from the system control panel
3. manually delete all remaining files in the firefox program folder, which usually resides in C:\Program Files (x86)\Mozilla Firefox\
4. reinstall firefox

I still get the error.

You did delete the Firefox program folder after you have uninstalled Firefox?

This usually happens because of a failed Firefox update that left you with a mixture of old and updated files in the Firefox program folder and best to recover from this is to delete the Firefox program folder to do a clean reinstall.

You can find the full version of the current current Firefox release (38.0.5) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/

I install Firefox it works initially then I close ans when I come back I get the error XUL runner Error Platform 38.0.1 is not compatible with 38.0.5 Please help me

This is a problem with at least the application.ini file in the Firefox program folder. You may have security software that is protecting this file and possibly other against changes.

Can you post the content of the application.ini file in the Firefox program folder?

Did you reboot the computer after removing the Firefox program folder?

What security software (firewall, anti-virus) do you have?