I dug into the code and tested again, and it appears that this bug/issue is actually around the event.shiftKey which registers as true in librewolf when the caps lock key is on, and shift is not pressed, and will register as false if the caps lock key is on, but shift is also on. Firefox on the other hand will register the event.shiftKey status as true/false simply based on whether the shift key is down or not, which is what I would generally sxpect from the browser.

In case it helps, the code in question is below:

if ((event.which == 83 || event.which == 115) && event.shiftKey) // shift-s
{
    // save the post.
}

This way, it checks for lower or uppercase S (83/115) and if the shift key is down. Thus the post will save with the shift+s key, no matter what state the caps lock key is in (in Firefox/chrome). However in librewolf, because the shift key is tied simply to capitalization (combos of shift and caps lock), if you just type a captial S, the save post functionality will trigger.