You could just upgrade your OS X to something newer.
You can still purchase Snow Leopard, and the last I heard, it's only $20, but could still be the original $30. Even so, that's not purchasing a new Mac, just a few quid to get your system updated, and you can then use that updated FireFox again
You have to call Apple sales direct, and talk to a sales agent to get it.
Update Snow Leopard to latest with Software Update, and you will then be able to purchase and upgrade to the current Mountain Lion, if you want to do that.
Safari supports 10.5 at the moment but what is happening is common in the trade.
As browsers are upgraded, they need to work with the latest versions ... and those are not always compatible.
If you upgrade to 10.6 you will have one of the best OS's around ... but that has a limited shelf life too. While it is supported by F.F, how long for is anyone's guess.
I think your MacBook pro with comfortably live with 10.6 and assuming it is an intel machine, should be suitable to go as far as Mountain Lion.
Incidentally, the general rule is ... Upgrades are paid for and updates are free.
As far as Browsers go, I;d try the PPC G4 version of TenFourFox is the most up to date browser for PPCs, they even have G4 & G5 optimized versions...
Should run on Intel upto 10.6.8 in emulation.
Sure, you can do that.
If your iMac is a year old, it came with Lion.
Upgrade to Mountain Lion on your newer Mac.
Make an installer partition on an external hard drive, or save the Mountain Lion install to a USB flash drive (8GB, super cheap these days)
You can upgrade any system that meets the requirements, and that you own with that same install for Mountain Lion.
Problem is - you can't upgrade the Leopard system directly to Mountain Lion, so you would need to also purchase Snow Leopard first.
OR, you can backup your files on the Leopard system. Do an erase and install of Mountain Lion on that older Mac, then restore your old files from the backup. Easy Peasy!
And, if you already own Mountain Lion, that makes it even better!
If you have upgraded to Mountain Lion, then all you need to do is download it through your App Store.
There is no extra charge to do this download, if you have already purchased Mountain Lion.
DON'T continue with the install.
DO download and run a utility that will easily help you make a drive, like a USB flash drive, into an OS X installer, that you can use on any compatible Mac: LionDiskmaker is really good, and easy to use. I would NOT suggest burning an OS X installer DVD. The utility can do that, but a hard drive partition or USB flash is MUCH better.