For quite a while, I was getting security nags from Firefox every time a web site wanted to run a Java applet. Firefox would tell me I needed to upgrade to the latest version of Java. That would have been fine, except that I was already running the latest Java (1.8.0_66 as of this writing). I manually downloaded and installed the SDK (which includes the JRE). So apparently there was something about the installation that was confusing Firefox into using an earlier version (?).
Anyway, long story short, I found a PPA from the ever helpful folks at Web Upd8 that makes it easy to get the installation correct, at least on Ubuntu and derivatives (including Mint). The PPA installs a package that downloads the binaries from Oracle, installs them, makes them the defaults etc.
On an unrelated note, this was not the post I started to write. I enter the math notation in this blog in LaTeX and rely on MathJax to translate it to MathML for browsers that understand MathML (Firefox being one). I noticed a week or so ago when viewing an old post that Firefox was inserting copious unnecessary (and unwanted) vertical space on either side of any line containing math, whether the math was "inline" or "display mode". Horizontal spacing was fine; only vertical spacing was munged. Checking around, I found that (a) it was happening on every page with MathJax content, not just on my blog, (b) it depended to some extent on which font was being used (with the XITS font, no extra space was inserted), and (c) Google Chrome was unaffected. So it appeared to be something involving the combination of Firefox and specific fonts. I did some research on Firefox extensions and was literally half way through typing up a post with a solution when, by chance, I discovered that the problem apparently went away. I don't know what changed, but suddenly Firefox is getting the vertical spacing correct again.
So my new tool for all technical problems is to write half a blog post about them and then see if that is enough to cure the problems. :-)