Since Firefox 136, pickers cannot be opened from non-focused tabs.
Unintentionally, background pages of extensions and devtools pages of
extensions were also affected, because their browsers are embedded in a
hidden XUL window, which is considered not focused.
This patch updates CanonicalBrowsingContext::CanOpenModalPicker to allow
pickers to be opened from specific hidden extension browsers again.
To evaluate the feasibility of enforcing the restriction for hidden
background pages, this patch puts the restriction behind a pref,
enabled by default on Nightly only. If no signs of broken extensions
are reported for a few releases, we can set the pref by default, to
enforce the restriction (and document the change accordingly).
After another few releases we can evaluate whether to enforce the
restriction permanently (effectively removing the exception from
this patch) or to revert the pref.
Differential Revision: https://phabricator.services.mozilla.com/D239046
2b9e9d3154