This certainly comes a little late, but it took me this long to finally look into it, and I'm glad I did, so let's share.
As you may know, with Firefox 24 the behavior of the "Click to Activate" feature
for plugins, most notably Flash, was changed. I don't remember when it was first
introduced, and am too lazy to go look for it, but for a little while know there
has been an option
plugins.click_to_play that one could/should set to
in order to have plugins not enabled by default.
And if/when you wanted to activate something, all you had to do was click on it. This would not enable the plugin for the whole page, but just that one element you clicked on.
As they explained, Mozilla did change this behavior - apparently too confusing for some users - and now there's a supposedly much better UI, I suppose.
Allow Now or Allow and Remember?
That is, when you click on a disactivated plugin, you get a little prompt asking you to "Allow Now" or "Allow and Remember" ? Which makes it pretty obvious what they'll do, right?
For one, they activate the plugin on the entire page, it's all or nothing, no more fine selection.
Also, "Allow Now" doesn't mean, as you could have thought, that once you leave the page/close the tab, the plugin is disabled again. Nope, it'll remain enabled now, for the entire domain you activated it for, and for quite some time. Which I suppose was made clear by the "Allow Now" label, or something.
Seriously though, turns out "Allow Now" means allow for the entire domain, and remain enabled for 60 minutes. As for "Allow and Remember" it's the same, only for 90 days. So "Now" does remember for a while, while "Remember" does forget after some time. That's what makes obvious sense for Mozilla I guess.
"Good news everyone!" Those can be changed: using options
plugin.persistentPermissionAlways.intervalInDays you can customize how long
the plugin will remain enabled/activated for in each case.
A nice trick
The cool thing being that is you set e.g.
plugin.sessionPermissionNow.intervalInMinutes to 0 (zero), then it won't
remain enabled at all. In other words, you really enable it for now and that's
it. Close the tab, leave/reload the page, it's not enabled anymore.
How about the per-element feature?
If you happen to have read the Mozilla blog post linked earlier, you'll already be aware of that nice little addon: Click to Play per-element which, as you guessed, restores the ability to only enable the clicked element, and nothing else.
So now, combining the two - which is what I do/recommend - you can have the choice:
Click on an element to only enable said element and nothing else (and only for now of course, close/reload and it's disabled again);
Click the icon on the left of the address bar, and click "Allow Now" to enable it on the whole page, but again only for now;
If you're mad, use "Allow and Remember" and be scared for the next 90 days (or however long you wish).
This pretty much restores the original behavior we had in earlier versions, which was - IMO - much better. Either way, you should now know all you need to set things up the way you want, hopefully.