Judging by the information you gave me, it sounds like Firefox has the problem, not your computer. If you may, can you send me some information about your Firefox?

Please click the orange Firefox button in the top left corner of the window, go to the arrow next to "Help", click Troubleshooting Information, go to the new tab that opens, click "Copy Text to Clipboard" button, open Notepad, then paste it into there and save it as "troubleshooting info", then send the file to (Email removed by mod). This is going to to provide me with some information to work with so I can resolve your issue. (No personal information is in the file, just things like Firefox's addons. Look for yourself) Thank you!

Post that information here so the rest of us can see it also. No personal information will be shown, only your system information.

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Clear the Cache Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

and

Remove Cookies Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

If there is still a problem, Start Firefox in Safe Mode While you are in safe mode;
Firefox Options > Advanced > General.
Look for and turn off Use Hardware Acceleration.

In cases like this you can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

You may have to clear the cookies from this website as well as posted above.

You can inspect and manage the cookies for the domain in the currently selected tab:

• Click the "Site Identity Button" (globe/padlock) on the location bar
• Click "More Information" to open "Tools > Page Info" with the Security tab selected
• Click "View Cookies" to view the cookies for the domain in the currently selected tab