xul.dll is a file that is part of the firefox web browser.
I suspect that it has been infected with some type of malware
and your anti malware program has deleted it.
Search you C drive and see if the file is indeed missing.
If it is not found the you can download a replacement from
a number of sites on the internet.
Copy the downloaded file to the firefox directory on your
drive, probably in
C:\program files\firefox
and see if that helps.
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