Have you tried: click Firefox button > New Tab > New Window OR CTRL+N OR File > New Window (if using the Menu Bar)

Once open, Firefox locks the Profile that is in use and you can not open another window with that Firefox version and Profile combination using the Windows Desktop icon or the Windows Programs list.

Yes I tried that. Unfortunately still not working. Cannot open another browser window. Only one window opens with multiple tabs. Any other ideas. Thanks.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.com/kb/Safe+Mode

• https://support.mozilla.com/kb/Troubleshooting+extensions+and+themes

Firefox 4 and later versions save the previous session automatically, so there is no longer need for the dialog asking if you want to save the current session.
Use "File > Exit" or "Firefox > Exit" (Mac: "Firefox > Quit Firefox") if you want to restore multiple windows.
You can use "History > Restore Previous Session" to get the previous session at any time.
You may need to click the orange/gray Firefox button to see History.
There is also a "Restore Previous Session" button on the default about:home Home page.

See also:

• http://kb.mozillazine.org/Session_Restore