The problem was the banking security program TrusteerRapport, which has apparently caused problems with flash in Firefox before. I stopped the process and the problem disappeared.
I will, of course send a report to Trusteer and have posted this info on the Mozilla site as well.
Since upgrading to Windows 8 Pro (with a downloaded upgrade, not a "clean" instal, so all my apps and settings were retained) I get the following error message when starting my customised version of Firefox with several pinned tabs, or when flash content loads in an uncustomised version in another user account, or when that version is closed if no flash content is loaded :-
(Title) - FlashPlayerPlugin_11_5_502_110.exe - Application Error
(Content) - "The instruction at 0x71a30000 referenced memory at 0x71a30000. The memory could not be written. Click on OK to terminate the program"
This error dialogue cycles repeatedly until the Shockwave plugin can be disabled or the tab closed. This also happened with the 11.4.402 version prior to today's update.
I have tried running the browser with add-ons disabled, and uninstalled/re-installed Firefox, but the problem remained. Can anyone tell me what the problem is and how to fix it?

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