Just had to re-install on Debian and it seems things have changed with the update to 13 "Trixie".
The good news is the bundled mspdebug is now the latest version so you can just install it with apt.
The bad news is that the library "libmsp430.so" which is needed to run mspdebug on the newer FET debuggers or Launchpad boards is no-longer bundled with Debian, nor does it seem to be available from the Internet anywhere (despite there being numerous projects and websites dedicated to building libmsp430 and the MSP430 most seem to be out of date, obsolete or simply silently fail...many of the downloads from TI just fail without any explanation, I suspect it might be that they are 32bit ELF *.bin files and they wont run on a 64bit system?).
Anyway, several hours of digging around and I found the only place where libmsp430.so seems to be available:
https://www.ti.com/tool/MSP430-FLASHERThis has the MSP430-FLASHER tools (make sure to download the 64bit version) which can be installed into your home directory and inside there is the libmsp430.so file. I simply made a symlink to it from /usr/lib/libmsp430.so but you could probably include the install directory into the search path for libraries or just copy the library over.