Ok, disabled the Java Development Toolkit plug-in, uploaded a couple more files to Box. No problem, no crash. Worked fine.
Unlike Joe, I have the option to enable the plug-in, so I re-enabled it. This time, after re-enabling it, I get a warning with a link to the page same as Joe's link, this one;
https://addons.mozilla.org/en-US/firefox/blocked/p428
It wasn't there before. But the plug-in is now enabled and the option button to disable it is still there. Still works, but with the warning.