In response to my comment, I did find this blog post which attempts to justify their behavior. To summarize, Dave is saying that essentially the entire application is the API, and to keep plugins working they would have to stop changing Firefox. I think it would behoove Mozilla to think this through a bit more carefully if it's not already too late to do so. They could (have) offered a stable guaranteed API that would not change, but still offered a volatile API (if that isn't an oxymoron). This way, add-on developers could partition their code and more easily find what might break during updates.

I've just about had it with Firefox. FF15's automatic update again broke at least one of my essential add-ons (Tabkit). Furthermore they have made it nearly impossible for a reasonable user to figure out how to downgrade. Mozilla's development behavior is reprehensible--it is reckless to repeatedly break API contracts with every release as they have. Their management needs to read a book on responsible API development. I am so ready to dump FF as soon as someone else develops a hierarchical tab plugin for an alternate browser.