It goes without saying that Adobe Flash is a mess on Linux. Several versions back Adobe changed flash so that a large number of systems no longer support recent versions of flash under Linux. The problem I discovered is that recent versions of flash only work on systems with a CPU that supports SSE2 instructions. You can find out what instructions your CPU supports by looking at the FLAGS when you run the command:
Note that the CPU must support SSE2 instructions, not just SSE instructions. This actually excludes a whole lot of useful Socket 462 (A) Athlon motherboards. Basically if you have a Socket A motherboard you can no longer run a recent version of Adobe Flash under Linux. It is possible to revert to an older version of flash, but the problem then becomes that many sites won't run without a newer version of flash (the chicken has destroyed all the eggs in the hen house).
Adobe Flash is a deal breaker for a lot of our clients. Some time ago I made the decision not to build Athlon-based boards anymore except for Athlon64 or newer. The decision had more to do with the speed versus naming convention than anything else. At the time I didn't know that the issues with flash were related to the SSE2 instruction set. If you're thinking of putting Linux on aunt Jane's old PC, use a live CD or Windows based tool like CPU-Z to check first to see if the CPU supports SSE2 instructions.