Could you check your plugin setting on Firefox's Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Plugins. Look for "Java(TM) Platform SE 8 U25". If you find that, check that it is set either to "Ask to Activate" or "Always Activate" and not to "Never Activate".

Note that Firefox is a 32-bit application. If you only installed the 64-bit version of Java, you probably also need to install the 32-bit version in order to get the plugin compatible with Firefox. See the note on this page: http://java.com/en/download/manual.jsp

I found the problem. Firefox is not compatible with Java 8. At least not with the websites I use. I re-installed Java 7.x, and it all worked again.

That's strange, I just installed Java 8 before replying to your post and it tested out fine on http://www.java.com/en/download/installed.jsp . But maybe some sites are testing for it in a way that Firefox doesn't support. ???

That address just checks the Java-installation itself. Try this one (it's in Norwegian, but you should see the applet anyway. It's the (un)employment and social welfare dept in Norway. No viruses)

https://idporten.difi.no/opensso/UI/Login?realm=norge.no&goto=http://idporten.difi.no/opensso/SSORedirect/metaAlias/norge.no/idp2?ReqID=s23172d79660c7de1ad8b3f0aa5315d92863b99116&service=BankIDDetector&ForceAuth=&locale=nb&gx_charset=UTF-8

Thank you for the example. The page uses an <embed> tag with:

type="application/x-java-applet;version=1.8"

But when I open the about:plugins page in Firefox (you can type or paste about:plugins in the address bar and press Enter), under Java(TM) Platform SE 8 U25, that content type is not listed -- it maxes out at

application/x-java-applet;version=1.7

But maybe that isn't the issue, because the Java plugin still prompts me for permission to run the applet, and it's only after that that I get the "plugin needed" message.

The Oracle/Java test site uses an <applet> tag instead of an <embed> tag so that might bypass this issue.

Further research is definitely needed.

That embed works for me with Java 8 (8u25) although I have to click and accept a lot security prompts to make the form appear (the second time is quicker: only two prompts).

If you have declined such a confirmation then you may have to clear the Java cache to get all alerts once again.