One of the latest casualties is Download Statusbar, which ceased working in Firefox 26. A supposed fix does nothing, as its reviews note (and if you take a glance at what the XPI file actually contains, you'll see why; it contains pretty much nothing at all, which coincidentally makes you wonder about the level of quality control on AMO), and the confusingly similar-named extension that still works with Firefox 26 is a poor replacement at best.
So why does Mozilla keep doing this? I can understand the desire to keep APIs clean, but users actually use extensions to do things they otherwise couldn't do (easily, or at all); breaking compatibility should only be done when unavoidable, after careful consideration.
Can you imagine having an OS that updates to new versions nine times a year, breaking random programs every time by changing its ABI, unless these programs happen to pay close attention and update every time they're affected by an ABI change? Of course not; the very thought is preposterous. It should be preposterous with Firefox, too.
And that's doubly true considering that not only are extendability and customizability part of Firefox's unique selling proposition, but the Mozilla team has also taken the stance that all sorts of functionality needn't be implemented in the core browser because someone can write an extension for it instead. Fair enough; but then don't go around breaking extensions, either.
Naturally, Firefox will neither warn you of an extension not being compatible before an update nor inform you afterwards, BTW. But that hardly matters, since you're expected to always use the latest version, anyway; older versions don't get fixes, even for security issues. (Granted, there's an ESR release if you know where to look; that one may buy you a year, if you're lucky.)
I'm actually tempted to switch to the Firefox 24 ESR release just to be able to keep using this extension; it's an invaluable part of my browsing. Alternatively, maybe someone'll fix it for good so it'll continue working with newer Firefox versions, but I'm not counting on it.
It might be a good idea, anyway, since it won't be long until there'll be another UI redesign (dubbed "Australis"). I'm skeptical about this one — it looks like another attempt to make the easy things "easier" for the elusive "braindead noob" user that may or may not actually exist, while making the hard things impossible for the power user that definitely does (and I'm putting the word "easier" in quotes on purpose, since I'm far from convinced it would actually be anything of that sort in practice), and I have little doubt it'll break all sorts of extensions again, especially UI-related ones.
Really, guys. You apparently consider "doing good" to be "part of your code", but maybe in addition to "doing good", you should also think about whether you're doing right. Because as far as extensions go, you've got quite some room for improvement.