This Mac tip is provided by Mark Greentree and was originally posted on Mark’s blog – Everyday Mac Support. For more of Mark’s tips visit his site, follow him on Twitter, or browse his archive of posts here.
Do you use multiple windows in Safari?
Perhaps you clicked on a link that opened a new window, rather than a new tab.
If this is the case then you can simply navigate to Window > Merge All Windows, from the Menu Bar as shown below:
This option will take all open windows and add them as individual tabs in a single window.