2017
FIR - You cannot downgrade Firefox 55 profiles - gHacks Tech News
Once a profile has been used with Firefox 55 (or later), it should not be used with previous versions of Firefox. IndexedDB, the (DOM) Cache API, Service Workers, and the asm.js cache will all fail to operate, generating confusing errors and causing portions of Firefox and some websites to break. The profile can be made operable again in older versions of Firefox by using the profile "refresh" mechanism.
FIR 57 + GREASEMONKEY - Greasemonkey Alpha versus Firefox 57 - chez DuckDuckGo
Greasemonkey 4 is available as an alpha version currently. Compatibility is restricted to Firefox 56 and earlier howeve
2013
FIREFOX - Profile Manager | MDN - Multiple installations de differentes versions de Firefox
(via)Firefox and other XULRunner applications store user settings and data in special folders, called profiles. Firefox provides a built-in applet to manage these profiles, but it will eventually be going away (see bug 214675), so a new standalone Profile Manager application has been created, which works with any XULRunner application, and has many features not found in Firefox's built-in version.
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