[These issues have been moderately covered in another thread of yours. I’m just posting here in case someone sees the comments in this thread w/o having read others.]
There’s nothing that actually prevents you from keeping any particular version of Firefox for as long as you want. You can change the update settings in about:preferences where those settings are located depends on the version of Firefox you’re using. You can set it to “never check for updates”. [more detail] The primary issues are security updates and feature updates.
It’s also possible to have multiple versions of Firefox installed on the same machine. Thus, there’s no need to actually uninstall FF57. This is discussed in a bit more detail in another thread of yours.