If you are running the latest version of Flash 10.1.102.64 then I'm guessing your problem is browser-related, since Firefox 3.6.24, Safari 3.1.2 and Flash work fine on my PowerBook G4 with OS X 10.5.6. The newest version of Flash won't work properly if the browser you are running is too old, like Safari 1.x or 2.x. Even the 3.1.2 that came with my retail version of 10.5.6 works fine.
The latest and final version of Safari that will run on Leopard 10.5.x PowerPC/Intel is 5.0.6 from July 2011 and you can download it here: http://appldnld.apple.com.edgesuite.net/content.info.apple.com/Safari5/041-1604. 20110720.2HQcx/Safari5.0.6Leopard.dmg
Apple is no longer developing Safari for Leopard, but Mozilla is continuing to develop updates for Firefox into 2012, with 3.6.25 coming in December 20, 2011 and 3.6.26 in January 31, 2012. Don't know where the cutoff is for 3.6 but the 3.6 is only a year old and I'm guessing not everybody is upgrading their browser that quickly.
If you want the latest Firefox 3.6.24, download it here: ftp://ftp.mozilla.org/pub/firefox/releases/3.6.24/mac/en-US/Firefox%203.6.24.dmg