XMarks is a service for synchronizing bookmarks between different browsers. If you don't use XMarks, maybe one of your add-ons thinks you do??

Could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

If this stops the annoying message, you may have an unwanted extension. You can investigate on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then they should be listed on the right side. Since Safe Mode disables them, you might be limited in what you can do here. To start Firefox up again in normal mode, simply exit Firefox and start it up again normally.

Since many bad add-ons install outside of Firefox and don't have a remove button, here's my suggested procedure for tracking down and cleaning up bad add-ons. I know it seems long, but it's not that bad.

(1) Open the Windows Control Panel, Uninstall a Program. After the list loads, click the "Installed on" column heading to group the infections, I mean, additions, by date. This can help in smoking out undisclosed bundle items that snuck in with some software you agreed to install. Take out as much trash as possible here.

(2) Open Firefox's Add-ons page using either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons
• in the Windows "Run" dialog, type or paste
  firefox.exe "about:addons"

In the left column, click Plugins. Set nonessential and unrecognized plugins to "Never Activate".

In the left column, click Extensions. Then, if in doubt, disable (or Remove, if possible) unrecognized and unwanted extensions.

Often a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any improvement?

(3) You can search for remaining issues with the scanning/cleaning tools listed in our support article: Troubleshoot Firefox issues caused by malware. These on-demand scanners are free and take considerable time to run. If they finish quickly and especially if they require payment, you may have a serious infection. I suggest the specialized forums listed in the article in that case.

Success?