You seem to have a version from October of 2006 = 4 years old.

If you are sure you have a newer version, see this:
https://support.mozilla.com/en-US/kb/Websites+or+add-ons+incorrectly+report+incompatible+browser

If that isn't the case, Firefox 2.0.0.20 is the last ever release for Mac OSX 10.3.9 and earlier versions. Get it here: https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/2.0.0.20/mac/en-US/

But that version is also blocked by many popular websites, like YouTube.
Basically you need a newer operating system to install a newer version of Firefox.

about firefox: version 3.6.11

about this mac: 10.5.8

i think i'm up to date but i'm also not great at this stuff.

thanks for the help/suggestions.

It looks like you are up to date as far as Firefox goes, your UserAgent is probably locked on that old version by an extension you have installed.
http://en.wikipedia.org/wiki/User_Agent

type about:config in the URL bar and hit Enter If you see the warning, you can confirm that you want to access that page. Filter = general.useragent. Right-click the preferences that are bold, one line at a time, and select Reset.
Then restart Firefox

aha - i did the reset user agent - could only reset general.useragent.override but that appears to have done the trick!!

thanks so much. i wish i hadn't waited so long to address this!

You're welcome!

Firefox 4 will be changing what can and can't be modified with the UserAgent, there's been too many application that mess with it for no reason except "vanity" - the "name" of an application that does that can be tracked in server logs. There are other ways a website can be made aware of which plugins a user has installed for purposes of delivering content, without "broadcasting" that information to every web site you visit.