Chrome isn't going to work as it's auto-updates Flash and there is no Flash version for 10.5
Safari now disables older versions of Flash
You can use Firefox and re-anable the 10.5 compatible version of Flash, however you must also install the NoScript Add-on for Firefox.
NoScript will disable Flash as you surf the web so it's not on all the time, you click the Temporarily Allow All and the Flash button on content to see it once you have trusted the site not to compromise your machine.
It's really a shame, 10.6% of Mac users are still on 10.5 and Apple, until just recently because of the 600,000 Mac strong Flashflake botnet, hasn't paid any attention to that large portion of it's user base since July 2011.
I guess Apple realized if a large portion of those 600,000 people were on 10.5, and come to realize their machines were compromized because of Apple's negligence in issuing security updates promply, that would affect their decision to buy a new Mac again.
The habit of Apple ignoring older operating systems also extends to third party software, like Flash, third party browsers and others who also need to have Apple correct their software mistakes in OS X.
So the reason there is no secure 10.5 Flash version is because Apple previously ignored 10.5, so Adobe and others followed along, it was only upon the Flashflake botnet that Apple then dragged 10.5 off the shelf and fixed it.
Ignoring security and stability updates for operating systems still in major circulation was a bad choice on the part of Apple, something I hope from now on they correct.
Harden your Mac against malware attacks
