Firefox update ruins everything
FIXED
This is bad news for long time users of FF with extensions that have been since abandoned or left as they were as they just worked. I have got an extension that lets me close tabs of the same domain, left or right of the current tab, blank tabs etc etc... it's very useful... but that isn't going to be compatible anymore as it hasn't been updated in 4 years 
I also got a proxy extension that looks really bad after the update (they released it early and was quick to give instructions to get the older version working after the backlash from 100's of angry users).
It is entirely possible to keep the older version of FF if you really need to. Mozilla do warn that security patches will not be available if you do not have the latest version... but at least your favourite extensions will still work for now:
From this article: https://support.mozilla.org/en-US/questions/1141234
http://ftp.mozilla.org/pub/firefox/releases/
It may serve as a temporary work around while the extension developers play catchup with the new way that extensions work - they have been given a few months notice and beta versions to develop their extensions on but not everyone has caught up.
I've just had the update and also lost use of Stylish, however searching for Stylish in the firefox add-ons and simply re-adding it (Along with re-adding the themes) got it back to normal 
@HarryB I reported to Stylish about it not working... here is their reply..
********************************************************************************************************************************************
I had to re-add the styles as their response advised you.
After re-adding the styles they seem to be working for me, however your screenshots show no styles and don't appear to cover trying to re-add them (Unless the error screenshot was when clicking the option button that gives the option for managing styles or creating a new one)
EDIT:
I have noticed that it's a little buggy.
After posting it doesn't always apply the styles, but a quick F5/refresh has fixed that so far so it's fairly tolerable.
@HarryB The seqence of shots, wass the process I took, after the FF downgrade. showe me maximum whtie space on Google search page... and also on Plusnet forum pages.
I clicked on the S for Stylish... and got the "error " notice.... then I clicke d on the OK on the subsquent pages, as anyone would, who wanted to "fix the error"...
But no page actually fixed it... and so I checked the Tools> Addons route, to uninstall, and then re - install Stylish... moving forward, I then clicked the Stylish icon, to re-install the styles... however, all I get is those same results..
as mentioned. it is purely academic now, as the replacement for FF downgrade and the replacement of Stylish, has solved the problem.
Thanks for you input though.! 
@shutter Just out of curiosity, is Cyberfox less of a resource hog than FF? I'm asking this as a friend of mine is running a 10 y.o. Dell laptop, and she is finding it very sluggish by midday, even though she is a VERY light user. I'm trying to find her a less hungry browser, and all options are being 
explored.