Get TenFourFox and basically continue using Firefox. It gets all the necesssary Firefox security updates and will use all the Firefox Add-ons. If you have an Intel Mac, it will run under Rosetta/PPC emulation, which shouldn't slow you down much, if at all--at least give it a try. All your bookmarks, everything, will still be there. It will use your current Firefox Profile.
Plug-ins like Flash are disabled by default, but they can easily be reenabled. Although I would leave out Java and use Flash with discretion (get the FlashBlock Add-on, which will set a placeholder for Flash content), since there are known Flash exploits for older versions of Flash.
TenFourFox home page
TenFourFox for G5 download