I already tried uninstalling and re-installing the AFAIK necessary components (screenshot 1, screenshot 2) (before, I had Java 6 AND 7 installed. I already tried leaving only Java 7, and now I am back to leaving only Java 6), but I cannot get it to work.
How do I get JRE up and running in Firefox?
UPDATE: I had so much trouble with Java in Firefox quitting working after upgrades, etc. that I gave up on Firefox entirely (my main purpose of using this Ubuntu machine is to safely run Java applications away from my main computer). I now run Opera and never had any problems again - it comes with Java, and it just works.