I've developed a couple of Firefox extensions. One of those was an e10s-compatible replacement for Vimperator, because Vimperator relied on XUL[0]. (Vimperator broke long before Firefox 57, because e10s broke backwards compatibility with some extensions long before Firefox 57 did).
The old extensions "API" was essentially a way to plug into arbitrary parts of the Firefox codebase. A Firefox developer has previously said that "[previously] the entire codebase was our extensions API".
That's not scalable or maintainable in the long-run, and I would assume that any software developer who's worked on a moderately-large project would appreciate that allowing arbitrary entrypoints and coupling makes it impossible to do literally anything without breaking some part of that extremely ill-defined API.
It's really, really unfortunate that moving towards a modern approach to browser extensions meant breaking work that people had put in over the last fifteen years, but... there's literally no other way to do it. Firefox was the first non-experimental browser to allow browser extensions, and there are both benefits and costs to being first-to-market. In this case, the downside is that they ended up accruing this technical (maintenance) debt.
As someone who's written Firefox extensions that are no longer available due to the switch, I'm disappointed that this had to happen. But as a Firefox user, I'm much happier having Quantum and Electrolysis (e10s), and if the tradeoff is between those two directly, I'd choose Quantum + Electrolysis over those extensions.
Or, to put in XKCD form: https://xkcd.com/1172/