You can notice by looking at the build time in the version string.

This makes it impossible to share a profile between two GNU/Linux distributions (or even the official binary and the distribution binary) without hacking with the current versions, even if they are effectively the same version: one will complain that the profile has been run with a more recent version.

(But I sure expect and want my distributions to rebuild Firefox - I trust Debian for making sure no bit of proprietary code is here, and makes tracking opt-in, more than Mozilla)