Browser startup checks firefox version for milestone

[shamblonaut]

I did some digging after posting the issue at zen-browser/www#528, and found that the whatsnew page opened on a milestone change (or more specifically, a difference in the browser's current milestone and the value of the preference browser.startup.homepage_override.mstone).

That preference currently holds the value of the current firefox version. I did some surface-level testing, and found out that the whatsnew page (the URL specified in the preference startup.homepage_override_url) only appears after an update only if the update also consists of an upgrade to the firefox version.

This was confirmed by going from 1.10.3b to 1.11b (which has a firefox upgrade, and opened the whatsnew page), and from 1.11.b to 1.11.1b (which did not have a firefox upgrade, and also did not open the whatsnew page). I did these specific updates using the flatpak cli, so I'm not sure if it is reliable.

I possible solution I can see is to set the browser.startup.homepage_override.mstone preference to the current version of zen (along with the startup page override in the brand patches):

pref("browser.startup.homepage_override.mstone", "%VERSION%");

I am doubtful whether this will work, as I do not know how milestone changes work with firefox and derivatives.

Here are some pages I referred to:

• https://kb.mozillazine.org/Startup.homepage_override_url
• https://kb.mozillazine.org/Browser.startup.homepage_override.mstone

This does not completely resolve zen-browser/www#528, but I believe it could be a step in the right direction for it.