I had a difficult time finding these, so I figured I'd post them for others. They are two separate bugs, but #1 can prevent you from fixing #2.

This post is to fix two separate flash issues:
#1: Problems clicking on buttons/closing ads/etc.
#2: Slow, blocky, choppy full-screen playback.

Note: You may not have both of these! These are separate issues, with separate solutions. I am just including them in one post for convenience, as I had both problems on my system. If you do have both problems on your system, you will need to fix #1 first!

Problem #1: Problems clicking on buttons/closing ads/etc
This fixes the problems with clicking on buttons in flash, closing ads, etc. It can interfere with changing flash settings, which you need to do in order to fix the choppy/blocky full-screen playback issue.
Step 1)
You need to edit a file as root. Open up a terminal, and type or paste the following line:
gksudo gedit /usr/lib/nspluginwrapper/i386/linux/npviewer
Step 2) You are adding the line "export GDK_NATIVE_WINDOWS=1" as the next-to-last line in the file.
The file will probably end up looking like this:
. /usr/lib/nspluginwrapper/noarch/npviewer
Step 3) Save, and close gedit. You will need to restart Firefox.

Problem #2: Slow/choppy/blocky full-screen flash playback
If you're getting slow/choppy/blocky full-screen flash video, this may help you!
More detailed instructions here.
Step 1) Right-click on a flash item, like a youtube video or advertisement, and select Settings
Step 2)
Un-Check Enable Hardware Acceleration, but don't close the Adobe Flashplayer Settings
Step 3) In a terminal, execute the following commands:
sudo su
sudo mkdir /etc/adobe
sudo echo \"OverrideGPUValidation = 1\" >> /etc/adobe/mms.cfg
Step 4) Recheck Enable Hardware Acceleration, and click Close.
Step 5) Restart your browser.

Hope this is useful!!