Regardless of the processor, Mac OSX 10.8 i not supported in the latest Firefox 49.0 version.

https://www.mozilla.org/en-US/firefox/49.0/releasenotes/

Ended Firefox for Mac support for OS X 10.6, 10.7, and 10.8.

https://blog.mozilla.org/futurereleases/2016/04/29/update-on-firefox-support-for-os-x/

"Firefox Extended Support Release (ESR) 45 will continue to support OS X 10.6, 10.7, and 10.8 until mid-2017, but this ESR release will be the last that supports them."

Also see https://support.mozilla.org/en-US/kb/firefox-osx on your options since you have OSX 10.8.5 and not 10.9 or later.

Not sure why you would get a message about PowerPC while on a Intel system.

THANK YOU "the-edmeister" and "james" for your VERY HELPFUL replies. Because of a piece of legacy software, that is no longer available and does NOT run on later Mac operating systems, I must keep an older Mac on an older OS. Yet, at times, when I am on that Mac, I would like to be able to use Firefox. BOTH of you provided me with an EXCELLENT, workable solution.

Again -- Thank you!

The ESR builds generally gets eight updates so Firefox 45.8.0esr will be released when Firefox 53.0 is unless Mozilla decides to extend support of 45 esr alongside 52 esr.