You can use Alt + Enter instead of Enter to open a link (keyword search) in a new tab.
You can also middle-click the green Go button to open the link in a new tab.

I'd prefer to make it an option to ALWAYS open in a new tab.

type about:config in the address bar. Click yes, I'll be careful. There's a search field at the top of that page. Search for

extensions.tabutils.openUrlInTab (If it's not there, see below)

The Options If it's set to true, it'll open searches in the address bar (wonderbar) in a new tab. If set to false, in the same tab.

For boolean options (true/false) double clicking on the value will change it. Or, and for all other options, if you right click on the value, it'll say "modify" or "toggle" at the top of the menu. Click that to change the value.

If that option doesn't exist: If it's not there, right click on the left where all the options are go toward the bottom of that menu and under the "new" option it'll open a submenu, choose boolean. It'll ask you to enter a value. See "Options" two paragraphs before for the meaning of the values.

Hope that helps!

PS - If it has no effect, and you restart firefox and the the value was reset, you'll need to change the value in the user.js file. Google for changing user.js in firefox to find how to do so.