I don't believe there's a supported way to do this. The right answer here is to uninstall it and downgrade to a supported ESR release, and make arrangements to manage the updating yourself in the future.
Firefox 51.x is a general release, and general releases only have security updates for the CURRENT major release version, with NO overlap. As with any internet-facing application, continuing use of an unsupported version is a grave security risk - in this day and time, outdated applications rapidly turn into potential backdoors. If it's necessary to maintain an unsupported version, you should take every possible measure to isolate and contain that version, such as running it in a virtualized environment that can only access an absolute minimum of whitelisted resources, or running it via some sort of thin client solution such as Terminal Services or Citrix with similar network isolation. ESR releases will ease the pain a little as they offer roughly a year of support and guarantee a minimum of 12 weeks of overlap between supported releases. Additionally, this list and development blogs are used to provide as much notice as feasible in advance of breaking changes. In some cases ESR releases also have small differences in behavior as necessary to support such things as managed configurations, centrally deployed extensions, and private extensions, or to provide more time for enterprises to adapt to breaking changes (for example 52 ESR allows administrators to reenable some NPAPI plugins that are now forbidden in the general release.)
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