I'm a constant critic of most things Mozilla (as well as an avid Firefox user... there's a lot of overlap in that venn diagram) and most of what this intro rails against resonates, but I'll never understand why they get hammered for not implementing vertical tabs.
This is a feature that a small but passionate subset of their users use, but it's also a feature that already has an extension that said subset of their users uses. Further, said subset has a strong overlap with the never-Chromium crowd, so they're not going to lose them any time soon. Out of the thousand and one things that Mozilla could be doing to make Firefox more competitive, vertical tabs is solidly in the second half of the list.