Have you contacted Malwarebytes support?

The file is constantly being updated by Firefox. So I suspect it is a firefox problem.

What is the name of that "file"?

The file is: C:\Users\ME\AppData\Roaming\Mozilla\firefox\Profiles\kyofddlz.default\prefs.js

The file is: C:\Users\ME\AppData\Roaming\Mozilla\firefox\Profiles\kyofddlz.default\prefs.js

That file will get changed occasionally it is your main list of Firefox settings. It is probably saved every time Firefox closes down.

It seems rather odd that you would be blocked from using Thunderbird , but then allowed to use Webmail. Are you talking about the same email address and account ?

Did you do as Fred suggested and contact the support site of Malwarebytes ? Presumably you are using their paid for software.

It is also probably worth scanning your computer with all the tools mentioned in our article

• Troubleshoot Firefox issues caused by malware

edit typo

Please note that the scanners may scan program files and report problems. Does the alert just list the file, or is there more information?

prefs.js is a text file and can be opened as such. You can look thru the file if you want, but don't change anything unless you are sure of what you are doing.

It seems rather odd that you would be blocked from using Thunderbird , but then allowed to use Webmail. Are you talking about the same email address and account ? It's the same account & email. I'm not sure if the spamlist issue is connected to the malware problem or not.

I opened pref.js with notepad++, but the invalid character in the message is probably unprintable.

Did the message state what line number? Can you post it? How many lines is the file?

CRbill said

I opened pref.js with notepad++, but the invalid character in the message is probably unprintable.

Give us a cropped screenshot?

Or tell us the preference name.

You could temporarily rename the file prefs.js to something like prefs.js.old whilst Firefox is closed. When Firefox restarts it will recreate a file with default settings. Or better still just create a new additional Firefox profile.

A user friendly method of looking at may be to copy and paste the problem lines to a new notepad text file and then open that using the Firefox add-on

• Hex Viewer

If notepad can not handle such a copy & paste you can look through the whole prefs.js file with HexViewer, its a bit cumbersome, but prefs.js is not a massive file.

OOPS Helps if I provide a link

• HexViewer https://addons.mozilla.org/firefox/addon/hex-viewer/

Thanks everone. I've come up with my own solution which does not involve Mozilla or my current ISP.

That's good to hear. Can you tell us what happen?