If you are looking for specific older versions of Firefox (v12 through v51) or the 52 ESR (Extended Support Release), you can find them through Mozilla’s official archive. These versions are often used for legacy software compatibility or for systems that require the NPAPI plugin (like Java or Silverlight), which was discontinued in newer versions. Direct Download Links (Mozilla Archive)
Legacy Plug-ins: Firefox 52 ESR is the last official version that supports NPAPI plugins (Java, Silverlight, Acrobat). Download Firefox 12-51 52 Esr
There are several reasons why you might need to download an older version of Firefox: If you are looking for specific older versions
What is Firefox ESR?
Key Features of Firefox 52 ESR