Firefox started throwing modal alerts when the connection is broken, how to return the previous behavior when connection errors were displayed on the page?

Question

Mozilla Firefox

I suspect that the topic should be discussed in the Mozilla community, but something does not occur to me how to google it.
Iceweasel 38.5.0, a few weeks ago there was an update (Debian 8) that broke the default connection loss behavior (error page), modal windows with alerts started to fall out instead. This also breaks the TryAgain addonThe that interacts with the error page. Even with a working connection, if the previous page was received by POST, then when going back, some kind of alert flies out and the transition is blocked (at least it used to ask "Go Yes / No"). I somehow endured it for some time, but today it just pissed me off: I have 100,500 tabs open, I write something on the Toaster, then the network drops and dozens of alerts start bombing from the browser. If the browser was written by schoolchildren, then they would code just such a reaction to errors; honestly, I did not expect such stupidity from Mozilla developers, especially since everything was fine with this before.
It must be configured by some key in about:config and the old behavior can be reverted.
Question: what key did they leave to disable their "brilliant"