how to open embedded links in mail messages with a tab rather than a window in Safari?

Two-finger tap, or control-click on the link, and from the resultant secondary menu, choose Open With > Safari. If you already have a single or multiple tabs open in Safari, another tab will be opened with the link from the email.

Tested: Monterey 12.2.1

I believe that opening in a new tab is the default, but check Safari preferences and change if necessary.

Also, in most cases where there is a similar choice between a new window or a new tab, if control-click to open you can temporarily use the other alternative by also holding down the Option key.

So if control-click, Open Link creates a new window, control-click, also hold Option and choose Open Link in new Tab

Try deselecting Automatically collapse tab titles into icons. Shouldn't make any difference, but it is Apple's design behind this. Otherwise, you shouldn't need to, but reboot and see if it corrects itself.

That’s weird. I take it that also happens with links elsewhere, of is it specific to links in Mail?

For example, if you drag an address from the Safari address bar to the Desktop to create a .webloc file, and subsequently double click this file to open it, does it also open in a new window instead of a tab?

One thing you can try is to delete Safari preferences, in case they have become corrupted.

Quit Safari.

In Finder, press Command-Shift-G and paste

~/Library/Preferences/com.apple.safari.plist

then drag the file out of the folder.

Start Safari again, and adjust the preferences to your liking.

See if it now works.

Safari Preferences > Tabs:

What if you choose "Always" instead of "Automatically"?

I have the same problem and followed the exact steps but it still always opens in a new window instead of tab