Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Does this also happen in a new window?

Try to rename (or delete) the xulstore.json file in the Firefox profile folder. You can use this button to go to the current Firefox profile folder:

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

Hi cor-el!

• While in safe mode I Went to tools -> add-ons -> appearance. There is only one theme (I never installed other themes). This did not fix the issue.

• I did not click the reset button -- this time. Did that already, but as soon as I re-sync with my firefox account it re-sync's the configuration that opens the embedded window.
• This happens in new windows, new tabs, AND across different computers through sync.

• I deleted the xulstore.json file, but as soon as I re-open firefox in non-safemode it resyncs and the embedded bookmarks windows re-appears.

Thanks for your help.

Turn off sync until the problem is solved.

Are you syncing any extensions?

Ubuntu has the habit of making a lot of changes to the way Firefox behaves.

You can delete the prefs.js file to reset all preferences to their default.

Since my last post I started removing older extensions that I no longer use. Not sure which one did it, but solved the issue.  :-)

Thanks for your help!

That was very good work. Well Done.