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.
Note that this workaround may have security implications on the system. Note that this will not help on Windows as Flash Player is hard coded to run in its own process. You also need to set to false if you do. Search for and change its value to a higher value. Type about:config and hit enter in the address bar. Update your operating system with the latest patches.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.