Do you have a Intel processor or a PowerPC processor as your user agent suggests and Mach-O would indicate an even older OS X version than 10.5 (possibly OS X 9).

• Mozilla/5.0 (Macintosh; U; PPC Mac OS X Mach-O; en-US; rv:1.8.1.3) Gecko/20070309 Firefox/2.0.0.3

Are running Firefox in some compatibility mode via Rosette?

See:

• https://support.mozilla.com/kb/Websites+or+add-ons+incorrectly+report+incompatible+browser
• http://kb.mozillazine.org/Resetting_your_useragent_string_to_its_compiled-in_default

• Firefox 4 versions and later do not run on a PowerPC Mac.
• Firefox 4 and later require at least OS X 10.5 and an Intel Mac.
• http://www.mozilla.com/firefox/6.0/system-requirements/

Thanks for your response--now I understand. Yes, my Mac is an older PowerPC Mac. I've upgraded Firefox to 3.6.20 and it's working fine. Not gonna buy a new Mac in this economy, ya know? I appreciate your help. Aloha from Hawaii.

You're welcome

For an unofficial Firefox 4 compatible version (TenFourFox) that runs on PowerPC Macs with OS X 10.4.11 or OS X 10.5.8 you can look at:

• http://www.floodgap.com/software/tenfourfox/
• http://code.google.com/p/tenfourfox/