Confirmed.....I'm guessing that you have your security settings notched up a bit and when Firefox closes it clears the clipboard as well as its' own buffers and caches.
I will poke around with this a little and see if there's a setting to allow the clipboard to remain populated without leaving all the other potential security issues wide open.
Have you tried this same situation in Safari yet to see if it may be there as well?
EDIT: Went back into Firefox and unchecked the "Permanent Private Browsing" option under the Privacy tab in preferences.....this left the clipboard still populated after shutting down the browser {the way it used to be in v4). FWIW, the clipboard is also emptied when navigating away from the page that you clipped from -- not just once you've closed Firefox -- if Permanent Private Browsing is selected. Safari leaves the clipboard populated on shutdown.
Thanks for figuring out what was up; I'd never have guessed that I had run into a preference issue...that depopulating the clipboard was a security feature. (Thinking about it, though, it's not at all a bad idea.) Thanks, too, for pointing out that the feature is
page, rather than browser session or even Website, specific.
(My confusion was compounded by the fact that even though I couldn't paste the data, my Butler Recent Pasteboards still displayed everything I had copied during my Firefox session...
probably an unanticipated second generation security issue which I'll report to Mozilla.)
After reading your response I looked in Firefox's pref pane and found a bunch of new (Edit: I think) features that were checked by default, and Permanent Private Browsing was one of them.
I had checked in Safari, but not having had Private Browsing enabled I was comparing apples and oranges. After reading your response, though, I enabled it and found that, as you've mentioned, Safari does not depopulate the clipboard.
Edit: Only 140 characters are permitted when reporting a Firefox bug...quite a challenge.