Comment 3 for bug 293170

I have this issue too. I am pretty sure the issue is with nspluginwrapper, and here's why:

I installed Adobe Reader 8 via the Medibuntu repositories, along with the Adobe Reader PDF plug-in which also uses nspluginwrapper to work on 64-bit systems. This plug-in also doesn't work after a while.

If you'd like, you could check to see if you have the same behavior as me:

 - Add Medibuntu repositories. (See: https://help.ubuntu.com/community/Medibuntu)
 - Open Synaptic package manager, and install...
acroread-dictionary-en and acroread-l10n-en
 (If you do not choose the language packages first, it will default to German when you select acroread)
acroread
mozilla-acroread

 - Open Acrobat Reader, go to Edit -> Preferences -> Internet and uncheck "Enable fast web view." (This isn't really necessary but it prevents an annoying situation where acroread is stuck using 100% CPU.)

 - Find a PDF online in Firefox and verify that it is displayed using the Adobe Reader plug-in.

 - Next time you find Flash not working, check and see if Adobe Reader is also not working. (See if you can find a case where one works and the other doesn't.)

After you're done, you can remove Adobe Reader, since it's not really useful for anything other than viewing PDFs in the browser (and since that doesn't work reliably...).