Say I start with a window with my mail. Then, I open up a tab to look something up on Google. Before, there would be two tabs at the top, one listing the mail, the second listing Google. Now, the mail disappears. And there is no way to tab back to the mail, except to delete the Google tab, or to open up a third tab with the mail.
Firefox 4+ versions use a new build-in home page named about:home with a Google search bar on it (no internet access needed to load that page).
That about:home page only shows some snippets and has a button to restore the previous session if applicable.
If you want a home page with extras like previous Firefox versions have then set another page as the home page like www.google.com
The incident with the disappearing home tab seems to have resolved itself. I can now open new tabs from the home page without losing the home page.
However, I am still having trouble with the new tabs. I just updated to Firefox 15. Now when I open a new tab, it shows me that page with none boxes showing recent pages I have used. Or, I can click on the icon in the upper right corner and open onto a blank page. But I cannot get my home page, gmail, to be the home page when I open a new tab.