I didn't try it, but ...
The XPI contains various version of the Flash DLL, that it will simply
copy into your Profile/extensions/ directory.
And I assume once there, it provides some method to select which version ends up being run. (You can do that now yourself from Addons Manager by manually copying DLLs & then disabling those you don't want to be active.)
Older versions were all named alike, NPSWF32.dll, so if doing manually, you would need to differentiate between the namings. (Note that the plugin need not necessarily be
named NPSWF32.dll, as NPSWF32_10.1.dll works just as well.)
Here, I've added a few of the more recent versions of Flash for you.