Mac Pro 2008 11.8/11.9 plugin failures

Hi, since the release of 11.8 I haven't been able to update my Flash plugins successfully, I've ran the uninstaller, tried manually deleting caches, library files etc. to no avail. The only thing that works is installing 11.7. Now Safari is blocking 11.7 and I'm forced to use Firefox If I want to access any flash based content.

My Mac is running Snow Leopard 10.6.8, system and third party sofware is up to date, the system is regualrly maintained and has no other issues beyond the sudden demise of flash plugins newer than 11.7.

It's now got to the point that every time I take a chance on a new update notification (that always fails), I simply drag the updated plugin to the trash and drag a working 11.7 plugin straight into my internet plugins folder, clean installs etc make zero difference.

Thanks in advance.

Nick