regression: cannot change home page in Firefox again (Artful)

As for the solution, could defaultPref() be buggy in Firefox?

If I test defaultPref() as opposed to pref() using a clean profile, the Ubuntu start page appears instead. If the "Ubuntu Modifications" extension is disabled, the Mozilla start page appears. It seems the default home page isn't being set properly.

This is what happens under Firefox 56.0.

I tried it in a VM and for me defaultPref() works fine in FF 56 both with and without the Ubuntu Modifications extension.

Never mind, I misread the comment.
Yes, the regular Ubuntu start page appears instead.

Tinkered around a bit more and it really looks there is some weird bug.
The Ubuntu Firefox modifications set the home page three times, each time with pref():
startup.homepage_override_url
startup.homepage_welcome_url
browser.startup.homepage

Would it be acceptable as an workaround still to use defaultPref() until this is fixed?
You would get the regular Ubuntu start page instead of the UM page, yet user choice is respected if one does change the homepage.
Or at least provide a fix in the Welcome application if that is possible

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

Another possible idea: maybe put an option into Ubuntu MATE Welcome to enable/disable the UM welcome page in Firefox?

This existing upstream Firefox report looks to be the closest to the problem - there are bugs with defaultPref() under Ubuntu.

https://bugzilla.mozilla.org/show_bug.cgi?id=947838

I have made the (technically correct) fix here - but the defaults will not work properly until the bug is fixed in Firefox:
https://github.com/ubuntu-mate/ubuntu-mate-settings/commit/13845909ab5c53ed1dd59a5a9a375e2a7c7d9028

Oh my... that Firefox bug looks rather ancient with not much happening on their end. :(

This bug was fixed in the package ubuntu-mate-settings - 18.04.1

---------------
ubuntu-mate-settings (18.04.1) bionic; urgency=medium

  * Add window buttons applet to Mutiny layout.
  * Conform to glib-compile-schema naming convention for overrides. (LP: #1577715)
  * Correct Super+L keybinding to lock the screen. (LP: #1728880)
  * Drop AppMenu integration for Firefox and Thunderbird. (LP: #1719134)
  * Use defaultPref() for Firefox defaults. (LP: #1721965) However, the
    defaultPref() functionality is broken due to this upstream bug report:
    https://bugzilla.mozilla.org/show_bug.cgi?id=947838

 -- Martin Wimpress <email address hidden> Wed, 15 Nov 2017 23:05:55 +0000

For me it not only resets the homepage but also makes FF lock up on the start up for a couple of secs. Looks as if FF is actually trying to connect to ubuntu-mate.org and fails or whatever. I didn't investigate what really happens tho.

Yep Web standards have become a HORRIBLE BLOATED MESS and browsers too as a consequence...

Re #10
Or it's a separate problem, not sure.