Could it be the Add-on bar? You can show/hide that bar by either:

• pressing Ctrl+/
• right-click a blank area of the tab bar
• right-click the "+" button on the tab bar
• View menu > Toolbars

You also might try using the Customize mode to see whether there is an extra toolbar down there, perhaps related to an add-on. See: Customize Firefox controls, buttons and toolbars.

Another possibility is that the file that stores your customizations has become corrupted. You can rename the file and customize from scratch.

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Switch back to Firefox and Exit

Pause while Firefox finishes its cleanup, then rename localstore.rdf to something like localstore.old

Restart Firefox. Any improvement?

Tried everything you said, the problem is still there.

In case one of your extensions is involved, could you test in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled (Flash and other plugins still run)

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?