newwin footgun: lock `browser.startup.blankWindow` = true

edge case?: see AF (arkenfox) https://github.com/arkenfox/user.js/issues/1618 (long read: ignore if you can)

tl;dr:

• AF uses RFP which has newwin protections (AF uses 1600 x 900 as maximums: FF has no crowd anyway)

• /* 4507: disable showing about:blank as soon as possible during startup [FF60+]
  * When default true this no longer masks the RFP chrome resizing activity
  * [1] https://bugzilla.mozilla.org/1448423 ***/
  user_pref("browser.startup.blankWindow", false);

• the reason for this was cosmetic: hide the (more noticeable on slower machines) delay and resizing etc

A user has reported a case where newwin is not firing (on app start, I did not push for menu>newwin tests). I can't replicate (yet). It doesn't matter what state the browser was closed in (xulstore). It may have something to do with trying to open larger than the screen: the screen is 1536 x 864 (1920 but at system scaling 1.25) and we're opening at 1600 x 900 (both of which are just larger than the res available), and this likely makes the OS/app resize to fit the screen - I do not think this is maximized - the decoration for maximze/restore is at odds. AFAICT the user does not use any tile/edge snapping utils.

given TB's settings are 1000 x 1000 and we have users with both width+height lower res than that, perhaps we should lock the pref to true (default) - it's probably not even maintained (it's hidden) and is likely to just be a testing pref during rollout. The problem is resolved when the pref is at default (true)

@ma1 for your newwin investigations