For me, the same symptoms were caused by apparent incompatibility btw Greasemonkey and another add-on, autoHideStatusbar, so you could try disabling other add-ons.

I just tried this in Shiretoko (4.0b2pre) and it’s fixed there, i.e. the plus sign is preserved. So you probably deduced the right culprit. :)

As the summary says: when a user opens a new, empty tab, it opens next to the rightmost of tabs currently open in the browser. This is inconsistent with the way links in new tabs are opened, which is to have them next to the currently active tab.

Steps to reproduce:
1. Open multiple tabs in the browser.
2. Select the leftmost tab.
3. Press Ctrl + T.

Expected result:
Have a new, empty tab next to the leftmost tab.

Actual result:
The new, empty tab opens next to the rightmost tab.