you won't be able to downgrade that version of the browser later on to an older version.
・・・it is especially important for users who plan to move to Firefox 52.x ESR
when Firefox 57 gets released in November as the profile won't work then anymore
if they have installed Firefox 55, 56 or 57 at one point in time.
In short: If you want to switch to ESR, it may be a good time to do so before the Firefox 55 release.
You may alternatively create a backup of the Firefox profile before you upgrade to Firefox 55,
and restore it after you install Firefox ESR on your system.
You cannot downgrade Firefox 55 profiles
https://www.ghacks.net/2017/08/02/you-cannot-downgrade-firefox-55-profiles/
【参考】
有名拡張機能や「Cyberfox」が終了 〜「Firefox」のレガシー排除といかに向き合うか
当面は「Firefox ESR」の利用がベターか。長期的な移行計画を
http://forest.watch.impress.co.jp/docs/serial/yajiuma/1048985.html
