I’m running firmware 1.9.0 on a Trezor One, which is the highest I can go to maintain compatibility with a certain wallet. Can I buy a Safe 3 and install the same 1.9.0 firmware on it, so I can replace my Trezor One?
no.
Trezor Safe 3 follows Trezor T versioning scheme (in fact the firmware is largely the same) and the lowest existing version is 2.6.3
OK, thanks. The instructions say that the wallet would also work with a Model T with firmware 2.3.0. Do you think that firmware could be installed on the Safe 3, or are the Trezor firmwares always device specific?
The firmwares are device specific.
If you absolutely need this to work on a TS3, your best bet would be using a custom firmware modified to revert whatever breaking change your coin is having issues with.
OK, thanks. How would one obtain a “custom firmware modified to revert whatever breaking change your coin is having issues with”. By the way, the coin is BSV and the wallet is ElectrumSV.
I know 
be advised that BSV is not officially supported on Trezor, which is also why this only works on older firmwares.
If you are asking, the answer is “hire a programmer to build you one”. The modification is not difficult, if I recall the problem correctly, but the whole process is extremely technical.
Thank you for your help. I really appreciate it!