Yes. I have:
- removed the commenting
- disabled tab mix plus, last pass, personas, and classic theme restorer (which still allows you to choose rounded tabs)
- auto updated to 23.5
No joy. Tabs are square.
Updated to CT23.8 - still square tabs.
So, I switched to FF30 beta like you. Lo and behold, rounded tabs! Then I re-enabled all my plugins and the tabs are still round. Definitely compatibility differences between FF29 and 30beta.