How to prevent content being displayed from Back-Forward cache in Firefox?

There are multiple caches involved. There's the browser's document cache (bfache), the browser's HTTP cache, and possibly intermediate HTTP caches.

The <meta> tags you show above have absolutely no effect in current Chrome or Firefox. They may have an effect in IE.

So chances are, your page is just being read from the browser's HTTP cache.

If you really want to send no-cache HTTP headers, you should do that. But they need to be actual HTTP headers: as I said above, the <meta> tag "equivalents" do nothing.

And, importantly, any other intermediate caches are not going to be parsing your HTML so might cache things if you don't actually send the right HTTP headers.

If you set Cache-Control: "no-cache, no-store, must-revalidate" to http headers the page won't be cached in back-forward cache.

Firefox also considers event handlers on beforeunload event as a signal to not store page in BFC, but Safari ignores such handlers, so it's better to set correct http headers to indicate the nature of the page content (cacheable or variable)