Well like I said in my first post in this thread I'll believe it when I see it. However with that said I do believe they have that as one of their goals with the new e10s model although I'm not sure they've specified plugins exactly (and plugins are very much different from all the other stuff they're doing with e10s I suppose). I do know they've said they plan to slowly roll out sandboxing and isolation of extensions/tabs with it coming in phases (i.e. first isolation, then sandboxing).
For 32-bit Firefox Flash is already isolated with a Chrome like sandbox (it does actually use Chrome's sandbox AFAIK) running at a low integrity level among other restrictions (it's not as restrictive as using Flash in Chrome though).
This does actually bring up a good question though. Plugins are very much different from tabs, web pages and extensions since they're not being ran with the browsers javascript interpreter/JIT. Will Firefox drop NPAPI support and move to the Pepper Plugin API (PPAPI)[1][2] that's currently used in Chrome and other Blink based browsers (e.g. Opera)? The PPAPI was designed to be ran in a separate process from the browser so it lends itself to sandboxing/isolation. All of the major plugins on Chrome use the out-of-process model for this and other reasons.
I suppose there are some issues with attempting to implement the PPAPI on Firefox since AFAIK it was designed with Blink in mind. I'm not even sure if a formal specification was ever published outside of the source code in Chromium/Blink as the PPAPI Wiki section mentions[1]. It'll be interesting to see what Adobe does with Flash in the future and if they ever completely drop support for the NPAPI architecture. Google also has the added advantage of having access to the Adobe Flash source code although this has increased security on all browsers across operating systems and not just Chrome[3][4].
It'll be interesting to see who blinks (no pun intended) first, Firefox caving in to implement it or Adobe forcing their hand to provide some alternative (or maybe none at all). Sure you can live without Flash for a lot of things but there are still sites which are reliant on it so could Firefox get away with simply dropping all NPAPI support without a valid alternative? I know they already plan to drop NPAPI plugin support save for Flash but is that just the beginning?
tl;dr I think Firefox has to have a new plugin model before we're going to see isolated/sandboxed plugins, possibly implementing the Pepper Plugin API (PPAPI) that Chrome and other Blink based browsers currently use.
[1]
https://en.wikipedia.org/wiki/Google_Native_Client#Pepper
[2]
https://en.wikipedia.org/wiki/NPAPI#PPAPI
[3]
https://googleprojectzero.blogspot.com/2015/07/significant-flash-exploit-mitigations_16.html (just one example)
[4]
https://googleprojectzero.blogspot.com/2016/03/life-after-isolated-heap.html