Here are several suggestions on how you should be able to solve the problems you mentioned.
#1. Scan for corrupted Windows files
There may be some corrupted Windows files. I suggest running the system integrity check using the Command Prompt.
1. Click the Windows Start button and type cmd into the search box and once you see a black Command Prompt icon in the search results, under the Programs section, right-click it and select Run as administrator
2. When the Command Prompt window opens, type:
and press Enter. Note that there is space after "sfc"
3. Wait for the scan to finish. It may take up to 1 hour to complete. That command is used to detect and automatically repair corrupted Windows files. Once it is done, restart the computer and check if that made any changes
#2. Manually check for new Windows Updates and install them
Installing the Windows Updates may solve many Windows problems. I suggest manually checking for new Windows Updates and installing them.
1. Click the Windows Start button and type:
into the search box and press Enter
2. When a new window opens, click the Check for updates button. Wait a while and you will be able to install all new available updates
3. It may take a while to finish. When done, restart your computer and check if there are any changes
#3. Check the Windows activation status
If Windows 7 had been activated before, but now displaying it is not genuine and asks for a new activation, please do the following:
1. Click the Windows Start button and then click on Control Panel
2. Click on the System and Security category and then click on System
3. Scroll to the bottom of the window. Check what is displayed in the "Windows activation" section. You may need to click on "Activate now" or "Change product key" to proceed with the re-activation. If asked to type a product key, just enter the same one you had originally used
#4. Web browsers check
Since you mentioned Mozilla Firefox won't open web pages, please start another web browser, such as Internet Explorer, and check if it will work. To do that, click the Windows Start button and type Internet Explorer into the search box and press Enter. Let me know the results.