Firefox 57 Not Asking To Save Passwords (certain sites only)

Make a bookmark with the following code. You click on the bookmark before you login to a site. Firefox will now allow you to save the password. Any Microsoft property won't allow you to save the password.

javascript:(function(){var%20ca,cea,cs,df,dfe,i,j,x,y;function%20n(i,what){return%20i+"%20"+what+((i==1)?"":"s")}ca=cea=cs=0;df=document.forms;for(i=0;i<df.length;++i){x=df[i];dfe=x.elements;if(x.onsubmit){x.onsubmit="";++cs;}if(x.attributes["autocomplete"]){x.attributes["autocomplete"].value="on";++ca;}for(j=0;j<dfe.length;++j){y=dfe[j];if(y.attributes["autocomplete"]){y.attributes["autocomplete"].value="on";++cea;}}}alert("Removed%20autocomplete=off%20from%20"+n(ca,"form")+"%20and%20from%20"+n(cea,"form%20element")+",%20and%20removed%20onsubmit%20from%20"+n(cs,"form")+".%20After%20you%20type%20your%20password%20and%20submit%20the%20form,%20the%20browser%20will%20offer%20to%20remember%20your%20password.")})();

OK so here is another answer I think is more convenient that the Javascript one.

First go to your Firefox components folder, by default in my case it's "C:\Program Files\Mozilla Firefox\components"

Now open the file nsLoginManager.js

Go to the function at around line 804 (varies with version of Firefox), that reads:

/*
 * _isAutoCompleteDisabled
 *
 * Returns true if the page requests autocomplete be disabled for the
 * specified form input.
 */
_isAutocompleteDisabled :  function (element) {
    if (element && element.hasAttribute("autocomplete") &&
        element.getAttribute("autocomplete").toLowerCase() == "off")
        return true;

    return false;
},

Now comment out the first 3 lines, so it ends up like this:

/*
 * _isAutoCompleteDisabled
 *
 * Returns true if the page requests autocomplete be disabled for the
 * specified form input.
 */
_isAutocompleteDisabled :  function (element) {
    /*if (element && element.hasAttribute("autocomplete") &&
        element.getAttribute("autocomplete").toLowerCase() == "off")
        return true;*/

    return false;
},

Save it, restart Firefox and you should be good to go...