hello, when it is appearing as infobar on top of the browser it is legitimate - see Some plugins used by this page are out of date for comparison. unfortunately your plattform (os x 10.4 on powerpc architecture) is no longer supported by apple or the most common plugin vendors, so it isn't possible to get the newest versions of flash, java etc. working on your system.

in case you want to turn off the messages about outdated plugins, enter "about:config" into the address bar of the browser, confirm the info dialog, then search for the preference named "plugins.hide_infobar_for_outdated_plugin" & double-click it in order to toggle it to "true".

please also note that firefox 3.6.28 was the last build that ran on a powerpc, however if you want to use an up-to-date browser based on firefox you can switch to "tenfourfox" - Firefox no longer works with Mac OS X 10.4 or PowerPC processors

Note that if you switch to TenFourFox you will not be able to use plugins such as Flash and Java. TenFourFox does not support plugins.