The last version of Firefox that ran on MacOSX 10.5.x was Firefox 16. Unfortunately, that version is not secure, so it is not recommended for daily use on the wild wild web.

Please see this article: Firefox no longer works with Mac OS X 10.5.

Firefox 16.0.2 is the last version that works on an Intel Mac with the OS X 10.5 operating system.

• https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/16.0.2/mac/

You can consider to install the last Firefox 10.0.x ESR version.

• https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/10.0.12esr/

Firefox 17.0x ESR has replaced the Firefox 10 ESR version, so you won't receive updates.

Firefox 17+ versions require OS X 10.6 or later, so if you want to continue using Firefox beyond Firefox 16 then you need to update your OS.

• http://www.mozilla.org/en-US/firefox/19.0/system-requirements/

then 16.0.2 support the latest add-on builder? I tried but it say i cannot test/install, still to incompatibility issue...anyway that i can fix it?

Hi siusoon, you may want to ask about the Add-On Builder, or search for past threads, on the separate forum on the Add-ons site:

https://forums.mozilla.org/addons/