I can't think of any reason for a new window to load well but not a new tab. It could be the second try effect. Firefox sometimes seems to give up on a site instantly, but if you reload, everything is fine. Could you try a reload in the same tab the next time and see whether that works? Not that it's a good workaround, but just to get a better idea of what's happening.

Sorry, I forgot to put that in my original post, I tried that multiple times and it would never load the page(s). I found it rather odd, too, that's why I posted on here to see if anyone else had run into this as well. It's only been happening since I upgraded to the latest version (I'm running Mac OS).

Thanks!