Look at the settings in Options > Tabs and make sure that the box for "Open new windows in a new tab instead" is checked.

Okay I'm sorry, I still don't understand. I once had it to were when I had a tab open on one window and then I selected in that window another link then it would automatically add another tab at the top and put me in that window but still keeping the other tab next to it. So I can pop back in forth between tabs....but my internet was messed with and put back to default settings. Can you walk me step by step on how to make sure I can get this back ?

This issue can be caused by an extension that isn't working properly.

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

• Do not click the Reset button on the Safe mode start window or otherwise make changes.

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