your system library is located in your HDD not your user folder that's user/library/internet plugins..
Yes, I'm aware of that.
open your HDD in finder and it'll be right there..
No it is not under /System/Library - but I think perhaps you are trying to make things more complicated that they really are.
Mac OS X is Unix based and Unix is really very simple. Unix uses the forward slash ("/") as the file system separator and the leading "/" is normally taken to mean to "root".
Unix (unlike Windows, VMS and some others) is a single-rooted operating system, which means that 99% of the time, there is no need to specify the disk volume, because when that volume is mounted, everything is relative to root ("/").
So when you tell someone to look under System/Library on any Unix or *nix based system, that implies either $HOME/System/Library (but that doesn't exist), or /System/Library (which does exist, but it's the wrong place).
Open a terminal and you can see Plugins under /System/Library
Code:
bash-3.2$ ls /System/Library/ | grep -i plug
Address Book Plug-Ins
KerberosPlugins
LoginPlugins
UserEventPlugins
But these are not the Plugins were are interested in here.
The ones we want are under /Library - not /System/Library
... and of course not $HOME/Library ether
The ones we want are under /Library ... like this:
Code:
bash-3.2$ ls /Library/Internet*Plug-Ins/ | grep -i plug
Flash Player.plugin
JavaAppletPlugin.plugin
Quartz Composer.webplugin
QuickTime Plugin.plugin
nsIQTScriptablePlugin.xpt
I'm trying to help you, but you want to argue.
That's funny, I'm trying to help you (and the thread starter), but you want to argue.

But thanks for your efforts.
I gave a screenshot that showed you exactly where it was. Try deleting the flash plug in and reinstalling my gut says there is an issue with you're current plug in.
Why do I want to delete my plugin for?
Mine is working fine.
But if I wanted to know if some process was holding a file open, I'd probably use "lsof" or something like that.
e.g.
Right now it tells me that these files are open for flash:
/Library/PreferencePanes/Flash Player.prefPane/Contents/Resources/FlashPlayerPreferences.png
/Library/Internet Plug-Ins/Flash Player.plugin/Contents/MacOS/Flash Player
/Library/Internet Plug-Ins/Flash Player.plugin/Contents/PlugIns/FlashPlayer-10.6.plugin/Contents/MacOS/FlashPlayer-10.6
From the full output of "lsof" we can also see the process name/id and the last two files are held open by Firefox ... as we might expect, but the first one is held open by the SystemUIServer process.
We can kill SystemUIServer if we want - it restarts again.
Then run "lsof | grep -i flash" again.
Hopefully it helps with the thread starters problem, otherwise he/she might need to uninstall/reinstall as you suggested.
TLDR version - open Activity Monitor and quit the SystemUIServer process - it might help.