I've reinstalled a backup I kept of FF 36 - my entire workflow has slowly been gutted over the years.
1) With FF 41 I could no longer set my New Tab page to my Home Page without an add-on. As of Firefox 57 the add-on became buggy if you regularly and quickly try to { Ctrl+T -> Ctrl+L -> Begin typing URL } due to limitations with the Web Extension API. [0]
2) Lost Tab Groups as of FF 45 (the add-on isn't a full replacement of old functionality)
3) Lost many, many, many addons with FF 57 including a very specific tab management that an addon called FireGestures allowed for: using a context menu to navigate my tabs.
4) Lost Bookmark descriptions with FF 62, also lost the Developer Toolbar
5) Now losing Live Bookmarks with FF 64
I no longer recommend FF as the "power user" browser but as the "I dislike Google and want a good browser that isn't Chrome" browser. If you're a power user of some uncommonly used feature, expect it to be removed at some point. I'd love to see how Pocket usage is measured and the statistics of how many people use that trash that they have shoved down users' throats since FF 38.
[0] https://addons.mozilla.org/en-US/firefox/addon/new-tab-overr...