I assume this means it’s compiling with Xcode targeting the M1. Firefox has never had to target aarch64 on macOS before.

It's long since targeted aarch64 on Linux though, so it's not too surprising to me that they have it ready so quickly.

Apple ARM dev kits have been available for a few months, also.

https://9to5mac.com/2020/06/22/apple-arm-developer-kit/

It means they recompiled Firefox for macOS/ARM64.

Correct. Just installed it on my Intel Mac running Big Sur (had to put it in my home directory; I already had non-beta Firefox in /Applications):

    file ~/Applications/Firefox.app/Contents/MacOS/firefox-bin   

    ~/Applications/Firefox.app/Contents/MacOS/firefox-bin: Mach-O universal binary with 2 architectures: [x86_64:Mach-O 64-bit executable x86_64] [arm64:Mach-O 64-bit executable arm64]

    ~/Applications/Firefox.app/Contents/MacOS/firefox-bin (for architecture x86_64): Mach-O 64-bit executable x86_64

    ~/Applications/Firefox.app/Contents/MacOS/firefox-bin (for architecture arm64): Mach-O 64-bit executable arm64

It means that M1 Mac's will not be translating x86 opcodes. Apple has done a very good job of making the transition (relatively) painless, but it's still not entirely trivial for many large projects, so seeing this many native binaries is quite nice.