After installation, open the Firefox menu (☰), go to Settings > General > Firefox Updates , and select "Check for updates but let you choose to install them" .
Modern Firefox profiles are often incompatible with older versions. If you plan to run an old version alongside a new one, it is highly recommended to create a separate profile Automatic Updates: Download Firefox 12-51 52 Esr
Final note Choose deliberately: nostalgia, speed, or steadiness each has a cost. Let your needs — security, compatibility, or stability — decide which door you open. After installation, open the Firefox menu (☰), go
: Choose between win32/ , win64/ , linux-i686/ , linux-x86_64/ , or mac/ . Let your needs — security, compatibility, or stability
This is expected. Modern HTTPS standards (TLS 1.3) and certificate authorities often reject Firefox 12-51. You can try enabling older TLS versions in about:config ( security.tls.version.enable-deprecated ), but this is a security risk.
Microsoft's SmartScreen often blocks old MSI/EXE installers. You will need to click "More info" and then "Run anyway." Alternatively, use a Windows 7 virtual machine.
However, if you are a museum curator running a digital kiosk, an engineer connecting to a 2010 medical device, or a retro gamer trying to open a Flash game from an old hard drive: