What version of Flash do you have installed?

Try to start Firefox in Safe mode. This mode disables all extensions, user customizations, hardware acceleration, etc. temporarily for diagnostic and troubleshooting. If Firefox works fine in Safe Mode, then this is likely an add-on issue. Restart Firefox in normal mode, then type "About:addons" (without the quotes) in the address bar. Disable your extensions one by one, restarting Firefox between each one, until you find the trouble maker. Once you do, you can disable that add-on and report the issue to that add-on's developer.

A few other basic trouble shooting things you can try is this: Run all Windows Updates, install all needed service packs, etc.

Update your graphics driver (Firefox uses your graphics card for some rendering, and an out of date graphics card driver can cause problems. https://support.mozilla.org/en-US/kb/how-do-i-upgrade-my-graphics-drivers.

Update all your plugins (Flash, Java, etc.): http://www.mozilla.org/plugincheck/.

Download and Install MalwareBytes Anti-Malware, run a full Scan. http://www.malwarebytes.org/. This helps check that there are no viruses on your computer causing issues. You can uninstall this program after you clean off any infections.

Adobe Acrobat 10.1.3.23 (4/4/12)

I am going to disable it for the time being as the PC is unusable once the plug-in takes control.

I had assumed that there would be a separate Adobe Flash plug-in - but apparently not.

It is a nuisance as I use Acrobat a great deal.

That is a Adobe Reader, which is used for reading PDF Files. If you type "about:plugins" (without quotes) into your address bar, press Enter, and scroll down, do you have a "Shockwave Flash" plugin listed? If so, what version is it?

Thank you. Yes, it is v. 11.2 r202

Can I separately disable it?

Yes. Type "about:addons" into your address bar, and press enter (again, without quotes). Then, inside that window, go to "Plugins". Find flash, and there should be a disable button.

Unfortunately there is no separate Flash plugin shown, only Adobe Acrobat 10.1.3.23. Hence why I thought I would have to disable Adobe completely.

I do not know how to remove Flash whilst keeping the pdf reader!

Well you should have Shockwave Flash listed there :/

If you want to uninstall Flash, you can always go into Start, then Control Panel, and uninstall it from your Add Remove programs window

Thank you so much. Un-installing via the control panel did the trick. I have now got rid of the Flash plugin in Firefox but still have my Acrobat Reader. I should have thought to go down that route myself!!

I was totally confused as to why I could not see Flash as a separate plugin on the enable/disable window, but that is the way it was.

Much obliged. I now trust that the annoying Flash plugin with it's attendant hogging of the CPU will be a thing of the past.

I have the same problem and it is Shockware Flash, v11.3 r300. It is also a problem on two different systems. What really happens on my systems, is I have firefox open at a site that will use Flash Player. The longer the site stays open, the higher the Flash player CPU cycles increase until it reaches about 74%. At that time, my cpu resource monitor shows 100%. One system gets there faster than the other, but, both exhibit the same increase in cpu cycles. I have the latest version firefox (v13.0.1) and the latest version Flash player plugin. I have some websites that I use that will not allow me to uninstall or disable the plugin.

What can I do to fix this unacceptable problem?