For quite a long time now I've just allowed Firefox to kill the Flash plugin while I'm debugging. Annoying, but not annoying enough to do anything about it.
However, with a quick settings tweak I can now leave Firefox hanging for as long as I like while debugging.
Simply navigate to
about:config in Firefox, continue past its warnings and then find (for Windows) dom.ipc.plugins.enabled.npswf32.dll in the list. Right-click on it and toggle it to false. For Mac OS/X and Linux the value to toggle is dom.ipc.plugins.enabled.libflashplayer.so. Also check that dom.ipc.plugins.enabled is on its default setting of false.
Alternatively, look for dom.ipc.plugins.timeoutSecs and change it to -1.
Newer builds of Firefox (3.7a1pre and above) only use the dom.ipc.plugins.enabled value, which is true by default. If you're using one of these versions just change it to false. If you only want to disable crash protection for Flash then manually add in an entry for dom.ipc.plugins.enabled.flash player.plugin.
Having just said this about newer build, during a quick test on 4.0b7pre Windows with default settings, the plugin hang for as long as I allowed it without being terminated.
For more information, have a look here.