I'm putting this up here so other Linux nerds don't spend hours figuring out what I have......
I have an old Athlon XP 2800 system knocking around which was earmarked for Freya (my 5 year old daughter.....running linux of course.... start them young I say ).... everything went swimmingly with the installation until I tried to use flash... no go, no error messages just "cannot load flash plugin".
Now I'm no stranger to hardware/software/operating systems, it's been my bread and butter for many many years.....(you don't need to know how long...but lets just say I'm an elder geek, in fact I was a geek before being a geek was fashionable....sigh)
I went through all of the hoops installing from different sources, using different browsers, setting rights on the libflashplayer.so file, you name it I did it ....nothing nada, same error.
I initially thought it was maybe because of the low memory the pc had, but no.....
it's a really really simple fix
cat /proc/cpuinfo | grep flags
which will look something like mine below except it won't have the SSE2 flag
fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow rep_good nopl extd_apicid pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy 3dnowprefetch lbrv
if it doesn't have SSE2 in it, you'll have to go back to the 126.96.36.199 version which is here
that will solve your problem, unpack that into /usr/lib/mozilla/plugins directory or wherever your browser is trying to load it from and it will work..... you can even just overwrite the newer libflashplayer.so file .....