Flash has annoying habit of closing fullscreen mode whenever “anything” happens. Like when wind blows through the window or there is an earthquake on another continent. No seriously, it closes whenever it looses focus. But that is still too sensitive. I can’t watch movie in fullscreen on one screen when working on another. It closes whenever I click on that second screen.

So here is a version of flash plugin that does not do that. It’s version 10.0.22.87 (latest at this time) with one byte changed (compare with original if you don’t believe me ).
NPSWF32_20100213

Axel Gembe made patcher that handles both ActiveX and Netscape Flash plugins. It automatically finds and patches them so it’s much easier to use and should also handle future 10.0 Flash versions. If any.
Download on author’s page: http://deve.loping.net/projects/ignoflash/

PS. You can still close fullscreen flash with ESC key or using any UI button that might be provided by flash player.

PS2. Maybe there is a native solution for that? Like some hidden ini tweak for flash? Speak up if you know about such.