Entering URL in address bar doesn't open new tab

[bethel95]

Update: I just did a test where I disabled VT, restarted Firefox, and tried again. As expected, entering a URL in the address bar opens a new tab. Here's where it gets interesting: Re-enabling VT without a restart gets VT working again without breaking FF. Unfortunately, VT reverts to the light theme (I prefer dark), and switching themes doesn't work without another restart (the tab background goes dark, but the text stays red, so it's unusable). As soon as I do the restart, the new URL opens in the current tab, again.

Original post: Starting about 3-4 weeks ago (possibly longer), entering an URL in the address bar causes that site to open in the current tab rather than opening a new tab. I am (and have been) using TabMixPlus, if that's a factor. This was working properly (i.e., opening a new tab) after I installed this extension back in early Dec.

I'm using FF 36.0 (just updated yesterday) on Windows 7 Pro x64 SP1.