Temporarily for Firefox 55, or generally; permanent download links for previous versions
With the new Firefox 57 released yesterday (2017.11.14) very many addons/extensions will stop working. Since many of them are important personal efficiency tools, and it may take a while to either get new/updated versions (if at all), or find alternatives, I suggest that PortableApps for a while provide an extra, direct download link to version 55 (not 56, as some extensions already stopped working in that version).
I suggest making a "custom block" in the sidebar on the main Firefox page: