It looks like your new tab is set for about:newtab which is the new thing in Fx12.
Quote from the install confirmation page:
Starting with Firefox 12, Tabmix default for "Load on new tabs" option changed to New Tab Page. When you update Firefox to v.12, it resets Tabmix option to New Tab Page and you have to make your selection again. You have a few options if you had the Blank Page option selected previously:
re-set Tabmix to load Blank Page.
use the New Tab Page for new tabs.
use the New Tab Page but disable it by clicking on the button on the upper right corner, this will turn the New Tab Page into blank page for new tabs.
(or setting browser.newtabpage.enabled to false)
set Tabmix to load the New Tab Page and set the address to about:blank.
Another words, check your settings for new tab and reset them to Blank tab: Events/New Tab > Load ... Blank page]. You may also want to check and reset Events/Tab closing > Closing Last tab > When i close last tab ... > Blank page.
Of course, that's if you don't want the new New tab Fx 'feature' (which is what i see on your shot). Even if you do want it, it worth a try. I suspect your setting has been reset and bkmrks now open in a new tab for you since there's no blank tab.