Mozilla Firefox not working with my Windows 7

Hello!
I bought a new desktop computer few months ago. Mozilla is working fine in the last few days. Recently I noticed it is accessing websites very slow and will eventually display “web page cannot can’t be displayed!” I want to know what is the reason of this? How quickly and permanent fix this problem? Can you provide me step-by-step solution for this problem? Do I need to return my desktop since I still have my warranty with it? Please help me make my browser work. Thank you!
