Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

Are you using the Back button when you get this message or otherwise refresh the current page?

You get an alert about resending POST data if you go back to a page or refresh a page that was previously requested from the server by submitting form data via a (hidden) POST form. Firefox can only make sure to get the same page by resending that POST form. Firefox doesn't know what that form data means, so Firefox asks for confirmation before resending that form data as such an action can cause you to repeat an action and buy another item or post a message another time. A way to prevent this confirmation pop-up about resending POST data is not to use the Back button, but to open links on a page requested from a server by sending a form with POST date in a new tab or window with a middle-click or hold down the Ctrl key (tab) or Shift key (window) and left-click the link. You can close that tab or window instead of using the Back button.

If you use a bookmark to open a specific page on the server then try to navigate to that page starting with the main page or the sign in page of the website in case this bookmark is currently broken.