(I assume by 10.5.7 you meant 10.7.5 in your report as
Thanks for the kind word. However, we don't support Intel
systems because there are substantial differences and it's already
a lot of work just supporting Power Macs on 10.4 and 10.5. (The
browser runs under emulation on 10.6, though officially we don't
support that either.) If someone wants to step up and maintain
that, we'll be happy to facilitate, but this is not likely to be
something I'm going to do myself.
That said, you do have one option: for 10.6-10.8, the Firefox 45
ESR will still run on your Mac and is supported until June 2017.
You'll have to downgrade a couple versions, so it doesn't have the
absolute latest features, but you will still get security updates
and the browser will be safe for use. You can get the ESR for free: