TL;DR We have decided to not (re-)turn on throttling of timeouts from
tracking scripts, and also remove throttling of timeouts from tracking
This feature was, in the beginning, only intended for tabs in the
background, but experiments were also conducted to see the effect of
throttling for foreground tabs. These experiments showed that users
became inclined to abandon pages to a higher degree than before, and
after letting the feature be turned on for Nightly for a period of
time, issues that would actually block user interaction were noticed.
Actually being able to resolve these issues for foreground tabs is
difficult, especially in light of how we schedule timeout events after
the rewrite that allowed us to only use a single timer per
window[1,2]. Even being able to solve the issues related to throttling
of background tracking timeouts would introduce unwarranted
complexity, and by instead removing throttling of timeouts from
tracking scripts we would reduce complexity of timeout handling.
The silver lining is that the importance of throttling of timeouts
from tracking scripts has been reduced since we started throttling all
timeouts from background windows using the background execution
budget, to the degree that we are confident that throttling of
tracking timeouts is not as necessary anymore.
 Bug 1363829: https://bugzilla.mozilla.org/show_bug.cgi?id=1363829
 Bug 1371787: https://bugzilla.mozilla.org/show_bug.cgi?id=1371787