When you restore a tab session, the page content is loaded from the cache. Sites can manage this with the Cache-Control header. Locally, you can use a private window, clear the cache on close or disable the disk cache (browser.cache.disk.enable set to false in about:config).

Probably better is to clear the disk cache when you close Firefox instead of disabling the disk cache to prevent Firefox from loading pages from the cache on the next start.

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings