I use multiple Tiddlywiki and WikiOnAStick files to store data, and those files rewrite themself on-disk each time an edit is made. Until recently, each time I made the first edit, the browser would pop up a "warning" asking if I wanted to permit the file to write to disk, which I would obviously answer with "Yes", and "remember this setting" (and then Firefox would forget the next time the browser was closed and later restarted).
Well, now with Firefox 16, they've "closed the security hole" that permitted those files to rewrite themselves on disk... but not completely. Finally, the files that I had repeatedly told older versions of Firefox to remember to permit to write themself to disk aren't asking anymore, and are just silently working. But, any new WikiOnAStick or Tiddlywiki are just patently refused (by Firefox) the ability to write whatsoever.
The Tiddlywiki guys have worked out a solution where you can install an extension to re-enable the save function for their files. It is nice that they were able to do that, but they shouldn't have had to in the first case, and this doesn't help for new instances of WikiOnAStick.
The selective permitting of previously known files to rewrite themself leads me to believe that there is a white list somewhere in Firefox that tells it that those files are trusted.
So, with that, I get back to the question. How do I add a file to Firefox's hidden white list of trusted files?
Some recommendations I've found involve installing an older version of Firefox somewhere to use just for WikiOnAStick (and maybe Tiddlywiki) instances. But, I'm not interested in having another browser window open all the time, and last I checked, Firefox didn't let you open multiple instances at a time. So this "solution" is a non-starter.
I figure there's some way of hacking the Tiddlywiki extension to also supply the save ability again to WikiOnAStick files too. But then I'm sure there are other similar projects that are now rendered valueless without the ability to save. The problem is in Firefox. That's what I want to fix. I don't want to have to "pop a pill" to "abate the symptom" (possibly causing some other symptom later).
 In the past couple weeks, it looks like whatever tolerance Firefox was showing for known files that I had self-edited has completely dried up, and the files I prefer to use most are now "stuck" in their last state.
I really don't want to have to go installing an old version of Firefox, or worse, whipping up a VM from an old, insecure version of Ubuntu or Vector Linux. But I will if I have to. And if I have to, I'll have even more reason to want to ditch recent builds of these browsers that seem to be forgetting or missing the point entirely.