Steps to repro:
- Check “Open all external links in a new tab” in preferences.
- Open an external link using a mouse - it opens in a new tab as expected.
- Open an external link using the keyboard. It opens in the current tab.
e.g. External link to SitePoint in this post:
Tested on Firefox 40 & Chromium Version 44.0.2403.89 Ubuntu 15.04 (64-bit), both on Ubuntu Gnome 15.04.