Just a heads up for those still using older PCs. Mozilla Firefox is dropping support for PCs that don't have SSE2.
If you are not sure about your PC, you can check with https://portableapps.com/apps/utilities/cpu-z-portable
In the 48.0 release notes, https://www.mozilla.org/en-US/firefox/48.0/releasenotes/
changed: "After version 48, SSE2 CPU extensions are going to be required on Windows"
The wording seems to imply that 'after' 48.0, Firefox will not support older PCs that don't have SSE2. May be a warning about 49.0?
The 'recommended' system requirements for 48.0, https://www.mozilla.org/en-US/firefox/48.0/system-requirements/
Pentium 4 or newer processor that supports SSE2
Anyway, if you must still use an old PC and want Firefox, you should probably download the PAc installer for the last Firefox version that supports your PC and be careful about allowing automatic updates. You might put your old version in a different folder so it is not automatically updated. Remember you will have potential security issues without updates.