revert removed pref

I had previously removed devtools.selfxss.count from the .cfg, but we need that cause it's a branding specific pref and therefore it's not set in forks, causing the console to break.

add in the devtools section of BROWSER BEHAVIOR:

defaultPref("devtools.selfxss.count", 0);

relevant issue -> librewolf-community/browser/linux#80 (closed)