Some script must be blocking the result from getting focus. Annoying. Perhaps only temporary?

As a workaround, you can retrieve the result using a bookmarklet, a snippet of code saved as a bookmark on the Bookmarks Toolbar.

javascript:var txt=document.getElementById("cwotbl").textContent; txt=txt.replace(/\s/g, ""); window.prompt("Calculator result", txt); void 0;

To create the bookmarklet:

1. Copy the above code to the clipboard (it's all 1 line).
2. Right-click your bookmarks toolbar and choose New Bookmark.
3. Click in the Location and paste the code.
4. Choose a short name for the bookmark and click Add.
5. Test on your Google results page. You can copy the result from the dialog.

If you find it too inconvenient, then we could try to develop a better workaround.

That is caused by using -moz-user-select: none; instead of -moz-user-select: -moz-none;
So Google needs to fix this for Firefox.

.cwmd { -moz-user-select: none; }

• https://developer.mozilla.org/en/CSS/-moz-user-select

-moz-none Non-standard
    The text of the element and sub-elements cannot be selected, but selection can be enabled on sub-elements using -moz-user-select:text .

Add code to the userContent.css file.

• http://kb.mozillazine.org/userContent.css

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the user profile folder.

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

@-moz-document domain(www.google.com){
 .cwmd { -moz-user-select: -moz-none!important; }
}