I have also had this issue. It isn't consistent, but it does happen a lot. I usually have to tap on something else (a link or something), return to the page I was on, and then it MIGHT give me the option to open the page I want in a new tab.

This is just one of a few issues that Firefox for iOS seems to be suffereing from. I have had to abandon it for another iOS browser, but I refuse to use that hunk of junk called Safari ....... now that really IS rubbish ....... but Firefox is following it closely at the moment.