Yes, it's about time to upgrade your browser to enjoy better browsing experience using the fresh-from-the-oven Firefox 3.5. You can upgrade to the latest one using automatic update by selecting Help > Check for Updates. It will download approximately 9,5 MB (i think it's downloading the whole components instead of the updated one as it is a major update).
Check the cool new features of Firefox 3.5 on the Release Notes and start downloading people
Be prepared as most of your addons will NOT be available after you upgrade as they are not listed as compatible with Firefox 3.5 (for now). It would take few days to weeks to get the new one compatible with FF 3.5. Anyway, it's good to know that some addons are already working with FF 3.5, such as Greasemonkey, StumbleUpon, Firebug, FireGPG, ColorfulTabs, Better GMail 2, and AdBlockPlus.
After you upgrade, check the About Mozilla Firefox and you will get something like this: