Fullscreen Flash Video With Dual Monitors Workarounds

Because a "security feature" users with dual monitor setup can't enjoy watching fullscreen flash video on one monitor while doing stuff on other, in other word you can't multitask.

edit the flashplayer binary in a hex editor.

To get hex editor:

sudo apt-get install ghex

Now:

sudo ghex /usr/lib/adobe-flashplugin/libflashplayer.so

Now search for _NET_ACTIVE_WINDOW and replace it with some other string of the same length, e.g. __ET_ACTIVE_WINDOW. Restart firefox and enjoy!

Thanks for Tom for this tutorial.

There are no permanent fix for for this only workarounds.

Or

1. FlashVideoReplacer

This is Firefox only workaround.
What this does is replacing flash video player and playing video via default player on your system like Totem or VLC.
This way video is played via other player and that player will make video fullscreen even you do other stuff on other monitor aka multitask.

FlashVideoReplacerer works with YouTube, Vimeo, Metacafe, Ustream...

Download FlashVideoReplacer

There is also Greasemonkey script, Youtube without Flash Auto witch is similar as FlashVideoReplacer.

2. MaximizeFlash

MaximizeFlash is a Google Chrome extension witch allows you to resize a webpage flash element to fit the Chromewindow size, on the same page or in a new tab.

Now when video is resided you can press F11 for full screen.  Video is now in "fullscreen" and fullscreen will not be interrupted if you do stuff on other monitor.

This only have one problem, sometimes Chrome don't respond to F11 (exit fullscreen) so you need to Alt +F4

Download Maximize Flash

There is similar extension for Firefox, Flash Game Maximizer

If you know better workaround for this issue post in in comments.