Your System Detail list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check its content with a plain text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Create a new profile to test if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

See also:

• https://www.malwarebytes.org/antimalware/mac/

Thanks for your reply. There are two user profile files - user.js and user.js~ - I quit Firefox and moved both of them out of the Profiles folder, then re-booted. When I re-opened Firefox, the same thing happened. The Criteo web site opens a tab. I am a bit nervous messing around with the Profiles and testing a new one.

I attached an image of the tabs showing the Criteo one that always opens.

Any other specific suggestions?

Thanks, R

That looks like a bookmark on the bookmarks toolbar. Right click on it, and select Delete.

Thanks, Fred for taking the time to reply. Unfortunately it is not a bookmark issue. The bug in question opens a new tab each time I start up Firefox into my Home Page. The image I attached to my last message may not be very clear, but it does actually create a new tab to the Criteo web site each time I open Firefox. I am still at a loss. R

Type about:preferences#general<enter> in the address bar.

There are two settings;

• When Firefox Starts

When the browser is started, what do you want to display. Many of us choose Show My Home Page.

• Home Page

When a new page is opened, what do you want displayed.

1. Use Current Page. Use what ever page(s) are open at that time.
2. Use Bookmark
3. Restore To Default. about:home shows a Mozilla home page with tools.

You can use any of these that you wish;

about:home (Firefox default home page), about:newtab (shows the sites most visited), about:blank (a blank page),

or you can enter any webpage or about: page you want.

Thank you, Fred. Just for the information of anyone who was in any way interested in this silly saga, I will explain what happened, which, by the way, your response led me to.

I had chosen the Current Page as my home page, but when I checked the web address for the current page, it actually contained two web addresses - the one I wanted and the other offending one from Criteo. I have no idea why that happened, but regardless, when Firefox opened my Home Page selection, it also opened another tab with the Criteo web page. Go figure!!

Thank you all for your help. R

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution more easily.

Thank you for contacting Mozilla Support.