Version 52 of Firefox is very special in several aspects: It is the last version before a major feature break, the last ESR before most ad-ons stop to work, and it is an unusual ESR in that it differs from the RR (rapid release) with same version number in several aspects.
This web page collects all what I currently know about these issues.
This is the last version that supports
If you still need any of these, you have 6 weeks to switch from RR to ESR (switching later from a higher version number back to 52ESR is unsupported, because it may cause problems).
The included updater will automatically migrate users of XP and Vista from RR to 52-ESR. But not users of plugins or XUL and SDK add-ons. They must act manually.
This is also the last ESR version which supports classic add-ons that have not been converted to WebExtensions.
Version 57 (November 14th 2017)) is planned to run only WebExtensions. If you want to keep using XUL and SDK add-ons a little longer, you must also switch to ESR now.
This is an ESR release.
The next will be 59, to be published probably end of February 2018.
But it is an unusual ESR, see below.
Current users of ESR 45 will be upgraded to 52.2-ESR, which is scheduled to be published on June 13th 2017.
Switching from RR to ESR should be done NOW, on version 52. Not later when add-ons stop working. Because Mozilla guarantees only compatibility in one direction: Firefox 52 ESR can read user profiles from Firefox 52 and older. But it may be unable to read profiles from Firefox 57.
Most plugins use the Netscape Plugin API (NPAPI). This is being phased out because it is insecure. The only exception is Flash, which will keep being supported for some more time (eventhough it may be the most insecure of them all).
Examples for commonly used plugins that are not supported any more are: Java, Silverlight, Google Hangouts.
If you still need other plugins than Flash, you can
But in Windows (not in Linux) it also depends on whether you use the 32bit or 64bit version of Firefox.
This list summarizes which version supports what:
The preference plugin.load_flash_only can be used to
This pref will not work in Firefox 53.
Some Java JARs can also be run using "java web start", which does not need the plugin. Example: the Rapla calender.
Usually an ESR is identical to the simultanously released RR.
But this time there are a number of differences:
This web page is based on several sources, none of which is on mozilla.com.
If you know a place where Mozilla officially publishes such info,
please tell me. That would save me a lot of time and guesswork.
Some of the sources are:
various Firefox users: Enterprise Mailing List (archive not publicly accessible)
Sören Hentzschel: blog post (German)
Chris Peterson: deleted tweet