Ya know I been working this one on an Acer Aspire 2 GB ram with the ARM processor, anyway running nightly (updated daily no pun) I run low on ram and CPU, it hates all the plugins and customization I did 255MB ram up to 500 something, but in the end, if I keep clicking the warning box goes away and the page works finally. Using the old version may resolve it, but you may also make your system vulnerable to attacks targeting security vulnerabilities that were fixed in the new version.
Side note: If you noticed Flash Player misbehaving after an upgrade, it is likely that the new version is causing the issues for you. If that is the case, try running Firefox in Safe Mode to see if that resolves the issue. Your best bet as far as I'm concerned is to make sure you have Flash Player, the graphics driver and Windows up to date.
REPAIR SHOCKWAVE FLASH PLAYER DRIVERS
You can get the latest drivers for NVIDIA and AMD by following the links in this sentence. Update your video card driver to the latest version.To do so, load about:addons, switch to plugins, and disable those that you do not need. Disable other plugins that you do not need, especially Silverlight.To do so press Alt, and select Help > Restart with Add-ons Disabled. The easiest way to run a quick test is to run Firefox in Safe Mode as this will disable hardware acceleration for that session.
REPAIR SHOCKWAVE FLASH PLAYER INSTALL