Why won't Firefox let me open a new tab?

I use the latest version of Mozilla Firefox and today it won’t let me open a new tab. A new tab will open only if I right click a link and select “open in new tab”. But If I just click the + button on the tab menu, nothing happens. Same if I right click a tab and select “open new tab”.

Has anyone had this issue? I’ve already restarted the browser and it didn’t work. I also checked my internet options but I haven’t changed anything.