The WebExtensions API from FF is compatible with e10s (Multi Process FF) so when WE API is released in a stable form (FF48) then the extension developers can move forward. WE API alpha is available now to developers in FF 47 but is still being worked on.
I personally have moved on from FF and don't plan to go back. But I do keep up with what is going on.
Jim 
Mozilla wanted to deliver E10s as early as Firefox 36, but things got delayed along the way and it was clear that it would take longer than expected to deliver the new feature to the stable version of the browser. [..]
Mozilla revealed in January that 40% of Firefox users don't use add-ons. If you add Windows XP users to that, and those who have used accessibility tools, you'll probably come close to the 50% of users that Asa Dotzler mentioned recently when he revealed the schedule.
This means that the major part of the "close-to" 50% will get E10S with Firefox 48, or at the latest with Firefox 49, and that the remaining percent won't get it with Firefox 49 but at a later point in time.
Not all add-ons are compatible with E10S, and while major ones are already, others are not right now and some will never be compatible (for example abandoned add-ons).
http://www.ghacks.net/2016/06/07/firefox-49-multi-process-architecture/
Having worked here in the Forums with the current Norton team since 2008 and having visited some of their technical facilities I think (b)
b) Symantec had a legitimate concern about the security of the Add-on SDK
is much more likely because, after all, that's what Identity Safe is all about .... and when it comes to allocating (rationing) development resources I prefer them to concentrate on what they are here for -- protecting my system from malware -- rather than a late in the game add-on for which there are alternative solutions.
I seriously doubt that many users bought Norton in its various versions just because it had Identity Safe and rejected what they believed was a better protection from another software just for that add-on. Identity Safe and the "tools" are just so much bunce / icing on the cake for which there are better and free alternatives available .....
PS For the uninformed:
huwyngr:
-- so even if Norton had finished and released their new version of their "Toolbar" it wouldn't have helped because FF don't have their bit ready until that further release?
Hi huwyngr:
I don't think that waiting for Firefox to support WebExtensions was Symantec's only option, but it might have been the more expedient decision.
Mozilla announced back in May 2015 that they they would be dropping binary XPCOM support as of FF 41 (released Aug 2015) in their blog entry Dropping support for binary components in extensions. Other popular Firefox extensions like the LastPass password manager and Adblock Plus ad blocker have continued to work in Firefox browsers while their developers prepare for Mozilla's August 2016 support for WebExtensions, and many add-on developers have migrated their old XUL-based add-ons using Mozilla's Add-on SDK API in the mean time.
Mozilla Developer Network (MDN) noted in their article Porting from Google Chrome that "WebExtensions are designed for cross-browser compatibility: to a large extent the technology is compatible with the extension API supported by Google Chrome and Opera. Extensions written for these browsers will in most cases run in Firefox with just a few changes."
I have no idea if Firefox has not had a working Identity Safe for almost a year because a) Symantec could not migrate their old extension to Mozilla's Add-on SDK, b) Symantec had a legitimate concern about the security of the Add-on SDK, or c) they decided it was easier to wait until August 2016 for WebExtensions support and release a ported Chrome extension that works in Firefox. I'm voting for c) 
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32-bit Vista Home Premium SP2 * Firefox v47.0 * NIS v21.7.0.11 * MBAM Premium v2.2.1
.... after FF48 is released on 02-Aug-2016 and add-ons built with Web Extensions (link is external) (i.e., the re-designed Norton Identity Safe for Firefox) are supported.
Thanks for the explanations -- so even if Norton had finished and released their new version of their "Toolbar" it wouldn't have helped because FF don't have their bit ready until that further release?
So Norton is better of waiting for anything else that may need to be changed .....
Apparently, there's a likelihood of extensions being broken. I think I'll disable auto update for the time being !
http://www.zdnet.com/article/new-versions-of-firefox-prepare-for-its-biggest-change-ever/
huwyngr:Starting in Firefox 47, that whitelist is removed. Mozilla notes that Adobe Flash is the only plugin left that will retain whitelist status while the activation status of all other plugins that were whitelisted previously are changed to click-to-play.
That's from the linked site. Anyone care to explain/guess what it means in the context of Norton -- if anything at present?
Hi huwyngr:
This announcement only applies to plugins that support online content like audio, video, etc. The Norton add-ons in Firefox are browser extensions and have never had the option to use "click to play" so I don't think this whitelist removal has any impact on Norton.
extensions vs. plugins : Find and install add-ons to add features to Firefox
click to play: Set Adobe Flash to "click to play" on Firefox
The major change for Norton extensions in Firefox will likely come after FF48 is released on 02-Aug-2016 and add-ons built with Web Extensions (i.e., the re-designed Norton Identity Safe for Firefox) are supported.
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32-bit Vista Home Premium SP2 * Firefox v47.0 * NIS v21.7.0.11 * MBAM Premium v2.2.1
Mozilla made available a whitelist for plugin owners which had the effect that these plugins were not set to click-to-play in Firefox.
Inclusion on the whitelist required that the company put a plan in motion to move away from NPAPI, for instance by utilizing HTML5 in the future for functionality.
Starting in Firefox 47, that whitelist is removed. Mozilla notes that Adobe Flash is the only plugin left that will retain whitelist status while the activation status of all other plugins that were whitelisted previously are changed to click-to-play.
That's from the linked site. Anyone care to explain/guess what it means in the context of Norton -- if anything at present?
That's an update (20.0 MB) for the current install of Firefox.
