I just read it's even getting worse with Firefox 57, which will be released in November 2017. That is, as far as compatibility with old add-ons is concerned.
It was quite annoying as it used to be, when your add-ons, plug-ins and other scripts, even the essential security ones from your anti-virus application and related, wouldn't work a lot of the time, because Firefox would issue updates every six weeks or so, and the add-on updates would always lag behind, rendering them of little use.
Now for security and compatibility reasons, Firefox is being completely retooled, and old add-ons will have to be replaced with the new standard of 'WebExtensions'. This means all old add-ons will stop working as of November, unless the developers have completely revised them. Firefox say that in the end, it will be a good thing, as nearly all "extensions created with the new standard won’t break in new Firefox releases".
But as it is, three-quarters of my add-ons are currently labelled 'Legacy' (= outdated), which has me wondering how many replacements I will have to be looking for!
Full announcement: https://support.mozilla.org/en-US/kb/firefox-add-technology-modernizing