search results are missing

hi guys - i use mozilla firefox as my browser, my home page is set to google....now when i type in something in google ie "unlock samsung smart phone", anything really just an examlple, i then go to a few of the results, then when i click back on my browser to the first page, the searchbar is empty and i have to type in again"unlock samsung smart phone", never use ti before, any ideas ?, its ok for a ssingle word but when i have to type a long sentance in its tedious lol..cheers