Unable to add search providers to Firefox

[nomike]

---+ Abstract
As long as the "Unity Desktop Integration 2.4.6" plugin is enabled I could not add search providers to Firefox.

I.e. when I want to add startpage.com and click on the "install" link for either "HTTP" or "HTTPS" nothing happens.
When I disable the plugin and restart the browser, it works.
WebDeveloper Console dislays the following error:
---SNIP---
[22:29:56.955] TypeError: window.external.AddSearchProvider is not a function @ https://eu.startpage.com/eng/download-startpage-plugin.html:2121
---SNAP---

When I enable the plugin and restart the browser it's again broken.

---+ Reproducible
Allways

---+ Steps to reproduce
1. Ensure that the "Unity Desktop Integration" plugin is enabled.
2. Goto https://eu.startpage.com/eng/download-startpage-plugin.html
3. Click on either one of the install buttons

---+ Expected behaviour
A dialog should pop up asking me if I want to add the search provider.

---+ Actual behaviour
Noting happens.

---+ Additional Info
This is also happening when trying to add a search provider from http://searchplugin.net.
It however displays the following javascript generated message:
---SNIP---
You need to be using a browser which supports OpenSearch

You can also install search plugins by using the drop down menu to the right of the search box.
---SNAP---

When you enable/disable the browser plugin you do not need to restart your browser for the effect to occur but you need to refresh the page. If you do not refresh the page, the behaviour won't change.

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: xul-ext-unity 2.4.7bzr13.04.15-0ubuntu1
ProcVersionSignature: Ubuntu 3.8.0-26.38-generic 3.8.13.2
Uname: Linux 3.8.0-26-generic x86_64
ApportVersion: 2.9.2-0ubuntu8.1
Architecture: amd64
Date: Wed Jul 17 22:08:35 2013
InstallationDate: Installed on 2013-07-04 (13 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
MarkForUpload: True
PackageArchitecture: all
SourcePackage: unity-firefox-extension
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Krzysztof Kosinski (tweenk)]

This bug still happens in trusty.

[Bryan Quigley (bryanquigley)]

I was unable to reproduce on Utopic but it does have a newer version of the integration plugin
Unity Desktop Integration 3.0.2