It is possible to have sub-tabs under firefox, to improve you browsing experience.
- To activate this option just type chrome://browser/content/browser.xul in the address bar.
- It is use handy if you have a small screen.
- Note that this feature is still under developpement and that there are some bugs.
- Note that you are not limited to only 2 rows of tabs, you can still expand it furthermore by typing the same code in the address bar of an opened tab, though it's use is quite questionable.