After doing a dist-upgrade Firefox got updated to the latest version, how can I go back to the previous version?
I'm not sure if Sean Patterson's answer is enough to prevent it from being updated by the AppCenter in the future.
Hence, I'd also mark the package as held back using ‘apt-mark’ command in terminal with root privileges.
For example, making your package say Firefox unavailable for install, up-gradation or uninstall, type:
apt-mark hold firefox
To make this package available for update, just replace ‘hold‘ with ‘unhold‘.
apt-mark unhold firefox