I think unstable and the fixed versions use the same Firefox package, so you wouldn’t gain anything. The difference is rather in libraries that get used and how the distribution does things. For example, the changes listed in nixos.org/manual/nixos/stable/release-notes#sec-r… just appeared mostly one by one for me; one day, I wanted to update my system and got the error that the fonts option got renamed, so I had to change my configuration.
The fonts.fonts and fonts.enableDefaultFonts options have been renamed to fonts.packages and fonts.enableDefaultPackages respectively.
While when using a fixed point release, these changes won’t happen. Only when you switch releases. That’s what “unstable” refers to.