Check the file name for capitalization or other typing errors.
   Check to see if the file was moved, renamed or deleted.

I have downloaded a new copy of Firefox and was not having an issue and now the shortcut leads to the that address and it says that it does not exist. I can open a new tab and use the browser but this is a nuisance and unexplained. File not found Firefox can’t find the file at chrome://ubufox/locale/ubufox.properties. Check the file name for capitalization or other typing errors. Check to see if the file was moved, renamed or deleted.