Starting from the release of Mozilla Firefox 5.0 the browser could no longer remember your last open tabs by default.
The reason for this is probably because the browser opens more slowly when it has to load multiple tabs
But if you find this feature useful, there is still a way to bring it back.
How to Make Firefox Remember the Previously Open Tabs
1. Go to Options
2. Click on Generals
3. Look for "When Firefox Starts"
4. Select "Show my windows and tabs from last time"
5. Click "Ok"
The next time you open Firefox it will load any tabs you previously did not close.
If you don't like doing this by default, there is still a way to open your previous tabs using the history.
1. Click on History
2. Click on Previous Session