Starting in Firefox 44, Conkeror will give this error at startup:

1454083323678   addons.xpi      ERROR   startup failed: [Exception... "Component returned failure code:
0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIXPCComponents_Utils.isModuleLoaded]"  nsresult: "0x80040111
(NS_ERROR_NOT_AVAILABLE)"  location: "JS frame :: resource://gre/modules/addons/XPIProvider.jsm ::
XPI_startup :: line 2596"  data: no] Stack trace:
XPI_startup()@resource://gre/modules/addons/XPIProvider.jsm:2596 <
callProvider()@resource://gre/modules/AddonManager.jsm:221 <
_startProvider()@resource://gre/modules/AddonManager.jsm:828 <
AMI_startup()@resource://gre/modules/AddonManager.jsm:1006 <
AMP_startup()@resource://gre/modules/AddonManager.jsm:2758 <
AMC_observe()@resource://gre/components/addonManager.js:58

This will cause some addons to not load correctly, such as ABP. The cause of this error is this line:

http://mxr.mozilla.org/mozilla-release/source/toolkit/mozapps/extensions/internal/XPIProvider.jsm#2596

It appears to be the result of a restructuring of the Firefox Debugger placing it in a new location that
may not be accessible from firefox -app.

This FF bug was opened in response to this issue:

https://bugzilla.mozilla.org/show_bug.cgi?id=1244163

This also has been reported in Debian at https://bugs.debian.org/813039 and I can confirm this myself, 
too.

A more specific issue for this bug:

https://bugzilla.mozilla.org/show_bug.cgi?id=1245251

There is a patch that one can apply if they are willing to recompile firefox.

Actually you can download a binary at the link above by clicking B then build, but Conkeror doesn't
currently run on the resulting binary due to other changes such as (I think) removal of array
comprehensions in FF46.

In the above message "the link above" should have been "the treeherder link mentioned in the link above."

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.

Looks like this fix is going to arrive in FF45.

https://hg.mozilla.org/releases/mozilla-beta/rev/0558da46f20c