That would be the most 'up-to-date' version. Mozilla offers every minor release of v52, the final one being v52.9.0 ESR. It isn't clear if you need that specific version or that was just what Ubuntu previously had in their repository. The directory of previous releases is here. You will likely need to get this in the form of a tarball from Mozilla. Keep the downloaded package so you can reinstall, just in case. After you download it and before you install it, disconnect from the Internet until you get it installed and have turned off automatic updates. However, when ESR updated to v60, it ignored my settings and updated anyway. I had my system locked into v52 ESR for the same reason as you, and thought I had disabled updates. There is some basic coverage of these options hereĪssuming you know all of the risks and are still intent on sticking with an old Firefox, read the part in the first link about disabling automatic updates. The popular options are Waterfox, Pale Moon, and Basilisk.

Just to toss this out there, there are some Firefox spin-offs that still support the legacy add-ons but are maintained so as not to be sitting ducks for exploits. Verify that your legacy add-on is still available and supported (many add-on developers dropped legacy support and either re-coded for the new standard or threw in the towel).

They even recommend going to a competitive browser rather than downgrade. They go through a page of reasons why it's a bad idea because it is insecure, and a host of alternatives. See this Mozilla article about installing an old version.

Firefox 52 esr download