We need the name of the files in question. Can you post the full message here?

The names of the files are included in the question! DSENGINE.JS and DSENGINE.CFG

those files are not part of any legitimate Firefox installation...

You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

BSM1 said

I am running Firefox Quantum 57.0.3 under Windows 10 64 bit. I have Malwarebytes Premium 3.3.1.2183 installed. During my last scheduled scan it identified 2 Firefox files as Potentially Unwanted Programs. These were: PUP.Optional.WinBing - DSENGINE.JS PUP.Optional.WinBing - DSENGINE.CFG Please identify what these files do, if they are necessary, and therefore what action I should take.

This was likely caused by installing AdAware or Web Companion from Lavasoft, which may be bundled with other software. None of this comes from Mozilla or the Firefox web browser.

https://support.mozilla.org/en-US/forums/contributors/712839

I used to have AdAware installed but a recent update of Firefox terminated this. AdAware does not appear in my plug-ins or extensions. I have now run AdwCleaner and it did not find the 2 files in question. They both have a date of 22 Dec 2017, so are recent. Do you believe it would be safe to delete them?

Yes delete them. I had the issue myself a month ago when a program I installed was not the program it said it was and I dug the program out after 2 days of it changing my search to bing. I also got the 2 files yesterday as they were just added to Malwarebytes. Please see my post there : https://forums.malwarebytes.com/topic/217648-firefox-57-false-positive/

Please let us know if this solved your issue or if need further assistance.

Pkshadow Many thanks for the info and links. I checked all the various links and it appears to be a generic problem. Last night's MBAM scan threw up the two files again so I have deleted them. I had run AdwCleaner earlier but had only removed items related to Adaware. So I checked for the presence of WebCompanion and that was hiding as well. So I have now run AdwCleaner again and removed everything it found. I have not had the problem with constantly reverting to Bing. The only other problem I have, which is probably not related, is that Word 2010 keeps corrupting normal.dotm, which is a pain in the ass! Once again, thanks.