Migrating data to the newer machine is going to pose a challenge.
A thunderbolt 2 to 3 adapter may work together with a thunderbolt 2 to Firewire 800 adapter and a Firewire 800 cable.
If it doesn't just a straight ethernet cable between the two machines will work better with a thunderbolt-3/USB-C ethernet adapter on the new machine.
Then the Migration Assistant on the old machine and Setup assistant on the new machine will allow brining over only 64 bit applications. 32 bit applications and drivers won't work. Selectively bringing over what you know works best, and you can just do Documents and Settings, and then see what applications are 64 bit and install them new on the new Mac.
https://roaringapps.com/
is a great place to find what versions are compatible with Catalina and later for good compatibility, and Apple Silicon for best compatibility.
Windows ARM is needed with Parallels or VMWare on the new machine in place of Boot Camp on the older Mac.
Printer/Scanner drivers may work if you used AirPrint compatible printer with. the Option key when you add the printer, as long as they are on the same WiFi network. Otherwise check for the same compatibility on the printer manufacturer website for Catalina or later.
An external DVD drive is needed for any old CDs or DVDs.
Your external hard drive running the older OS will not be able to boot the newer machine, until you upgrade its operating system to be the same as the newer machine.
Firewire is not able to boot newer computers. It is best to adapt any hard drives to USB if possible. If you have a Firewire hard drive case, there are many USB-SATA adapters for old hard drives that can come useful. Do not move them out yourself unless you are acquainted with the anti-static routines for opening drive cases. Consult with an authorized service provider in person to ensure you don't mess yourself up.