Firefox 36 breaks Xubuntu 14.04, and how to fix it

Update (from Xubuntu change log):

Changes for firefox versions:
Installed version: 36.0+build2-0ubuntu0.14.04.4
Available version: 36.0.1+build2-0ubuntu0.14.04.1

Version 36.0.1+build2-0ubuntu0.14.04.1:

  * New upstream stable release (FIREFOX_36_0_1_BUILD2)
    - see LP: #1429115
    - revert removal of the deprecated -remote command line option, as some
      older software (eg, Python 2.7's webbrowser module) still depends on it
      (LP: #1425972)


Original Post:

In Xubuntu 14.04, the update that bumped Firefox to version 36 broke the ability to click a link in a message in Thunderbird and have the link open in Firefox. Instead Firefox would open, but on its default page. Mozilla helpfully documented that they dropped the highly obsolete -remote command line switch in this release, which previously provided a needlessly complicated way to tell Firefox what url to open.

How Thunderbird opens web links is pretty obscure. I don't think I ever messed with this before, which matters because how to fix it may depend on whether you were relying on defaults, or had messed with some setting in this area before.

Command line:
$ xdg-mime query default x-scheme-handler/http
exo-web-browser.desktop

Examining exo-web-browser.desktop in /usr/share/applications reveals that it runs a binary called exo-open. exo-open is an XFCE application, so it is safe to assume that it was last updated when dinosaurs walked the earth and should never ever be used for any purpose.

Command line:
$ xdg-mime default firefox.desktop x-scheme-handler/http
$ xdg-mime query default x-scheme-handler/http
firefox.desktop

And now clicking links in Thunderbird once more opens the links in Firefox, as it did before the update.