to be clear, this was not a decision made by librejs - all add-ons that are compatible with firefox versions less than 57 are not compatible with firefox versions 57 and greater - AFAIK any addon that claims to support both pre-v57 and post-v57 firefox versions is doing so by offering 2 distinct add-ons: one of the pre-v57 (XUL) type, and one of the post-v57 (WebExtensions) type - it would be nice if both version could be supported but there is not enough developers on the project currently; so the focus is on making one good version rather than 2 half-baked ones
the legacy version of librejs is now, and will always be available on the GNU FTP server: ftp://ftp.gnu.org/gnu/librejs/ - if you are using a browser based on firefox version less than 57 then there is no need to install any new version of librejs - the version you have will continue to work for as long as you keep it - if you want to use the WebExtensions version of librejs on debian today, using a browser that is officially supported by debian today, then you can get firefox v61 from debian today: https://packages.debian.org/sid/firefox as a side note, another browser you (and other users) may find interesting is the new iceweasel-uxp from the hyperbola project; which is the default browser for the hyperbola system - it is a fork of the firefox fork: 'basilisk'; which means that it will continue to suport legacy add-ons indefinitely, but also is FSDG-compliant like iceweasel and icecat - if enough people choose the option to continue using legacy add-ons, then perhaps it may become justified for librejs to put some attention into maintaining the legacy add-on; but it is not cler today how much demand there is for that
