The obvious fix I tried was to create an override in chrome.manifest to replace XPIProvider.jsm with a
patched version, but unfortunately chrome.manifest doesn't support overriding resource:// URIs.

However, I was able to resolve the problem by using a Firefox nightly (the fix has now been committed to
the main tree), after removing the use of array comprehensions from Conkeror's source code, as contained
in this pull request:

https://github.com/retroj/conkeror/pull/10

I can confirm that addons do work with a Firefox nightly as of today.

It does look like unfortunately we'll have to rely on nightly builds of Firefox for a while.