> The whole parade of extension signing, deprecating XUL, privileged extensions and XPCOM are all necessary to make sandboxed e10s processes work? I thought the child processes already are somewhat sandboxed right now.

Reaching into content windows is forbidden in multiprocess Firefox. That alone is going to break tons of addons. This necessitates a redesign. Since a major redesign is necessary anyway, it makes sense to future-proof the architecture so that addons will work in perpetuity. Ultimately, this ends up being friendlier to addon developers, since addons will break once instead of again and again as the architecture evolves.

> Wouldn't it be sufficient to write safer, less exploitable software? Isn't that part of the goal of Rust for example?

A Rust-based browser engine will never replicate XUL and XPCOM, because no browser engine besides Gecko can replicate XUL and XPCOM. That is because XUL and XPCOM are too ill-specified and tied to the exact internals of Gecko. As long as add-ons are written to an unspecified, ad-hoc API, it'll be impossible to change the underlying technology, preventing adoption of things like Rust. In the end, that holds back browser security.

You make a good point that software not written in C++ might need less security hardening elsewhere.

But sadly, the reality is that Firefox, Chrome, Safari and Edge are all massive C++ codebases, with lots of security problems. Until we fix that, browsers will need to protect their users as best they can, and that means removing dangers like unsigned addons, overly-flexible addon APIs, and so forth.

Perhaps some users would be ok with a browser with less security and more flexibility. But by default, browsers should be safe, since 99% of users don't understand software and much less software security.