You can do the same with Firefox. Simply use Beta, Dev Edition or Nightly. The first two are very stable. You just need to flip `xpinstall.signatures.required` to `false` in about:config.
As far as I know, installing unsigned extensions is also possible in ESR builds, in Firefox from Linux distributions, in Unbranded Builds from Mozilla (but I'm not sure if they keep older versions). In forks and in your own Firefox builds (from source).