They disappeared recently, about the time I updated to Firefox 12.0 (I haven't been changing any settings lately). A tab used to appear for each web page I visited, that stayed visible until I closed it; now there's only a tab for the page I'm on.

I tried a little cross at the top left labelled 'open new tab', but I couldn't see anything happening. A little down arrow next to it, called 'list all tabs', just tells me what web page I can see in front of me (however many I've visited in the session). Tools - options - tabs has the option 'Open new window in a new tab instead', but it has no effect that I can see.

Is it some setting I could do/a feature of FF 12/me being dim/a combination of these? I can still use History to go back to recent pages, but it means quitting the current page. A task I want to do needs 2 web locations open at once.

Under History I can see 'Recently closed tabs' and 'Recently closed windows' options, but they're greyed out.

Vista SP2, Adobe Flash Player & Foxit Reader were also updated recently. I use AVG Anti-Virus Free Edition 2011 (updating automatically) & Windows firewall.