Since upgrading toFirefox 4, I can no longer return to my e-mail message from an on-line attachment without shutting down the browser since there is no "close tab" option

Formerly, an e-mail with a website, photo, etc. attachment that I opened required only that I click close tab return to my e-mail message. Since upgrading a few days ago, the only option present is to exit, which now requires re-opening firefox then my e-mail browser. Any website I am examining that I want to close automatically exits me from Firefox.