FF29/30: top of tabs is not an active zone that allows switching
Since version 29 there is this additonal border around tab that prevents switch tabs.
Moving the mouse to top of screen when FF is maximized, one cannot switch to another tab directly. The cursor needs to be within the tab for it to react.
https://i.imgur.com/X9tKBQx.png
On the screenshot: say I want to go the 3rd tab, the whole area above it won't allow switching. I need to click just below it.
I'm using ColorfulTabs plugin if that matters.
Wubrane rozrisanje
Your addon has changed the tabs so that would appear to be a side effect of the addon.
Try temporarily using Firefox in safe mode to test that out. Do NOT use the RESET.
- Use Use NewButton
→ Help 
SafeMode (Restart with Add-ons Disabled ...) - See Diagnose Firefox issues using Troubleshoot Mode
Wšě wotmołwy (3)
Wubrane rozrisanje
Your addon has changed the tabs so that would appear to be a side effect of the addon.
Try temporarily using Firefox in safe mode to test that out. Do NOT use the RESET.
- Use Use NewButton → Help
SafeMode (Restart with Add-ons Disabled ...) - See Diagnose Firefox issues using Troubleshoot Mode
That's a bingo!
With Colorful tabs disabled behavior is back to normal and no more additional space above tabs.
Glad it helped.
Have you tried upgrading by the way you seem to be on Firefox 28 still and Firefox 30 is now out.