hello, can you try to replicate this behaviour when you launch firefox in safe mode once? if not, maybe an addon is interfering here...

Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

I started in safe mode, but there still was no way to open a new tab.

ok, you could also see if the options described in Navigation buttons like back, home, bookmarks and reload are missing help. if everything else fail you could use the ctrl+t keyboard shortcut to open a new tab as well...

Try to click the Restore Defaults button in the Customize palette window if you do not see the plus button on the Tab bar.

• 3-bar Menu button > Customize

I don't find a customize palette window. In the customize feature, there is no restore defaults button.

The Restore Defaults button is positioned in the bottom right corner of the "Additional Tools and Features" window.

Restore Defaults does not restore my Open New Tab feature.

Try to reset the browser.uiCustomization.state pref on the about:config page via the right-click context menu to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

I reset the pref, but nothing changed.

Create a new profile as a test to check if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox