PageMod add-on not triggering in recent Firefox versions

Add-ons Development
#1

One of my old add-ons, Ad LImiter, isn’t working on some computers. The symptom is that the add-on installs properly, but the PageMod doesn’t fire for some matching URLs on some machines. This used to work reliably.

It only fails on some machines. The add-on works with Firefox 38.0.5 on four machines I have: my own Windows 7 64 bit, Windows 7 32 bit laptop, Firefox 38.0 on Ubuntu 14.04 LTS, and Firefox 35.0.1 on an Android phone.

But if I try Firefox 38.0.5 on Windows 64-bit on a machine at another location (one of the machines at TechShop), the add-on silently fails to fire on relevant pages. I’ve tried installing the beta and nightly builds; they fail in the same way. But under the developer build (FF 40.x), it works on the same machine where 38.0.5, beta (FF 39.x), and nightly (FF 41.x) fail. I tried a second machine at Techshop configured slightly differently (running as user, rather than administrator), and FF 38.0.5 failed there, too.
Turning on the web console and all available logging doesn’t produce any useful error messages.

On my development machine, where the add-on works, running under “cfx run”, I’ve seen these messages:

[Child 4932] WARNING: pipe error: 109: file c:/builds/moz2_slave/rel-m-rel-w32_bld-000000000000/build/ipc/chromium/src/chrome/common/ipc_channel_win.cc, line 333
(This is mentioned in a Google Chrome bug! https://code.google.com/p/chromium/issues/detail?id=224891)

JavaScript error: resource:///modules/WindowsPreviewPerTab.jsm, line 406: NS_ERROR_FAILURE: Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsITaskbarTabPreview.invalidate]
(This may be related to https://bugzilla.mozilla.org/show_bug.cgi?id=1127577)

This isn’t an add-on conflict, I’ve tested with fresh installs with no addons and with fresh profiles.

Any ideas on how to chase this down? Because it works in the developer build, the better tools in the developer build don’t help.

#2

Try testing in the failing machine using Nightly with e10s turned off. If it works, then my guess is that something was fixed in 40 that is broken in 41 (possibly because of e10s). That should help narrow down the problem.

#3

Found the problem. There were two problems.

  1. One user had their browser in private browsing mode, but didn’t know it. The add-on is not enabled in private browsing mode. If Firefox is launched with “/private” as a command line option, all pages are in private browsing mode, but there’s no purple mask icon. If the Windows shortcut contains /private, that happens. See https://bugzilla.mozilla.org/show_bug.cgi?id=906117 where it’s discussed whether invisible private browsing mode is desirable behavior.

  2. The other user is in Europe. If you try to reach “google.com” from some countries, you’re now redirected to the country-specific version of Google to allow for country-specific censorship. This is a recent change by Google. The user was being sent to “google.ch”, which the add-on does not recognize.

So it wasn’t a page mod problem at all. Thanks.