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This JavaScript function isn't working in FireFox

I'm currently developing a website and I have this snippet of Javascript code which works perfectly fine in Google Chrome, but doesn't execute in Firefox. When I remove this function, the other functions below this one works fine in Firefox so I believe it's not a Firefox version issue. (I've already updated Firefox to check).
My question is this. Is there anything obvious about this function which prevents Firefox from executing it?
Please let me know if you need more context to the code.
Thanks.
// go over each filter button
filterToggles.forEach(function(toggle) {
let attrVal = toggle.getAttribute(['data-filter']); // find the filter attr
let newVal = attrVal.replace(' ', '-'); // hyphenate filter attr val
toggle.setAttribute('data-filter', newVal); // set filter attr with new val
});

depending on the version of firefox, forEach may not work.
https://caniuse.com/#search=forEach

How to remove linebreaks in IE with Javascript?

I am trying to remove linebreaks with JavaScript. It works in all browsers except Internet Explorer (tested in ie7, ie8). I have read a lot of posts but couldn't find a solution to the problem.
Var someText = "Here’s your text.\n It has line breaks that need to be removed.\rUsing Javascript.\r\n";
someText = someText.replace(/(\r\n|\n|\r)/gm,"");
What else can I try?
* EDITED *****
Here's more code and a better explanation of my problem:
function checkLength(fn) {
var fn = document.getElementById(fn);
var fnb = fn.value;
var fnb = fnb.replace(/(\r\n|\n|\r)/gm,"");
var len = fnb.length;
...
What I am trying to do is calculate the number of chars in a textarea. I had to calculate with both Javascript and PHP and because of the linebreaks, PHP and Javascript never came to the same number. When removing the linebreaks, it is all good except in Internet Explorer (when I calculate it with Javascript). fnb.replace doesn't change anything in Internet Explorer for the character count so that is why I am sure it does not remove the linebreaks correctly. In all other browsers it is fine, I can see the difference in the counter after removing the linebreaks in javascript. Only in ie it doesn't change a thing. I have tried a couple of things as well as your suggestions below and the char count before removing the linebreaks and after is the same in ie.
///////// MY ANSWER /////////////////////////////////////
function checkLength(fn) {
var fn = document.getElementById(fn);
var fnb = fn.value;
var fnb = fnb.replace(/(\r\n|\n|\r)/gm,"");
var len = fnb.length;
...
Like Tomalak said, my logic could be improved - Sorry for being new to JavaScript & programming. Maybe Tomalak doesn't make any mistakes but I'm sure everyone else does. We have to make mistakes to learn.
Internet Explorer didn't like
var fn = document.getElementById(fn);
var fnb = fn.value;
I had to change it to:
var fnb = document.getElementById(fn).value;
Even if it wasn't logical, it should have worked. It did work in all browsers except ie. It's a bug.
That was the answer I was looking for.

Try this:
someText = someText.replace(/[\r\n]+/gm,"");

In fact, the regular expression works just fine in at least IE8.0.
The real problem is that you wrote Var instead of var; Firefox lets you get away with it, but IE doesn't.

You don't need the m flag, but you ought to replace with a space instead of nothing.
someText = someText.replace(/[\r\n]+/g,' ');

IE Javascript error "Object doesn't support this property or method" within jQuery

For some reason, I am getting the following Javascript error in Internet Explorer 8 on line 3156 of jquery.js (version 1.4.3, non-compressed version): Object doesn't support this property or method. No error occurs in Firefox and Google Chrome.
This is the line the error occurs on:
if ( (match = Expr.leftMatch[ type ].exec( expr )) != null && match[2] ) {
Investigation (console.log(Expr.leftMatch[type])) produces the following interesting result: In Google Chrome, it outputs
/(^(?:.|\r|\n)*?):((?:[\w\u00c0-\uFFFF\-]|\\.)+)(?:\((['"]?)((?:\([^\)]+\)|[^\(\)]*)+)\3\))?(?![^\[]*\])(?![^\(]*\))/
However in Internet Explorer this is the output:
function() {
var p = this;
do p = p.previousSibling;
while (p && p.nodeType != 1);
return p;
}
On which exec cannot be called (it is undefined). The quoted function is not present within jquery.js. Does anyone have any clue why this happens, or what I can do to solve it?
I have, unfortunately, not yet been able to create a simple script to reproduce the problem, although I did find this post of someone having the same problem, but it does not offer a solution (the last post suggests the page should be run in Standards Mode, but mine already is).

As it turns out, I managed to figure it out by myself after several painful hours. It appears the Sizzle selector engine breaks in this unexpected way (and only in Internet Explorer), if you have defined Object.prototype.previousObject elsewhere.
Removing that declaration, or renaming previousObject to something else fixes the problem.
The funny thing is, I even put that code there myself (the Object.prototype.previousObject = [the function in my question]), but I did not recognize the code.
Well, that's another day full of development potential wasted.

I have discovered the same behaviour occurs if you attempt to add a method called "inherited" to the Object.prototype, ie Object.prototype.inherited = <some func>
It affects IE6, 7 & 8 but seems to be fixed in IE9 (beta)

May be to late to respond but I had the same problem and solved with selecting elements with plain java script rather then jquery!
var div = document.getElementById("myDiv");
var rect = div.getBoundingClientRect();
This works some how!

Javascript if statement refuse to load correctly from exterior script IE6 IE5.5

I see a very funny behaviour in my page when it comes to IE6 and IE5.5. I have a script (supersleight if you know about it) that puts PNG's back in business when dealing with IE6 and IE5.5. During execution of this, I want to change the background into using the Explorer alpha filter (if Javascript is turned on, use filter, otherwise stick to solid white).
I do this by:
if(document.getElementById('transparency') != null)
document.getElementById('transparency').style.filter= "alpha(opacity=60)";
...transparency is the id of the object in question.
Putting this at the end of the HTML page (or anywhere after 'transparency' was initiated) results in the script working. Putting it at the very end of the exterior script (deferred) however results in the filter NOT being applied.
However, when I remove the if statement and just tell the browser to use the filter it works (however only a few of the pages has got the 'transparency' id).
I tried to apply the if statement differently by using an alert box and trying both != null and == null and I get nothing.
This made me very curious so I tested this:
var tt = 5;
if(tt == 5)document.getElementById('transparency').style.filter= "alpha(opacity=60)";
Which gave an even stranger result with an error screen saying
tt is undefined
All of this runs perfectly in IE 7 and above...
I realize this is really two different issues but still...
Can anyone give me a clue as to what's going on?

Does this work?
var t = document.getElementById('transparency');
if (t && t.style) t.style.filter="alpha(opacity=60)";
How about this?
try {
document.getElementById('transparency').style.filter= "alpha(opacity=60)";
} catch (e) { }

A weird regex problem

The following code results in "undefined" for lastIndex:
var a = /cat/g;
var l = "A test sentence containing cat and dog.";
var r = a.exec(l);
document.write(r.lastIndex);
However, it works perfectly for r.index (and r.input).
I am using Firefox. Does anybody have a clue?
EDIT: OK, the above code works perfectly in IE! Further, in Firefox, it works perfectly if instead of calling r.lastIndex on line 5, a.lastIndex is called. It appears that Firefox does not return lastIndex property in the result - rather sets the property for the pattern invoking the exec() only. Interesting that IE sets both.

This is one of those places where Microsoft decided to add some stuff to the language and act as if it was supposed to be there. Thankfully, they are now cleaning up their act and documenting such nonsense.
To be clear: Firefox is correct according to the ECMAScript Language Specification 3rd Edition (PDF, 705KB).
IE is not correct; its behaviour is a proprietary extension. There is no reason to believe that this IE-specific behaviour will ever be supported by any other browser. It certainly isn't at the moment. See JScript Deviations from ES3 (PDF, 580KB, by Pratap Lakshman of Microsoft Corporation) Section 4.6 for more on this particular deviation from the spec, including tests showing no support on other browsers.
Note also that this may not even be supported in the future by IE: a number of proprietary IE CSS-related mechanisms are disabled by default on IE8-in-IE8-mode, and future implementations of JScript may find a reason to similarly disable this extension to the language.

lastIndex is a property of a RegExp object. So try this:
a.lastIndex

To avoid all the weird, try this
var a = /cat/g;
var l = "A test sentence containing cat and dog.";
var r = a.exec(l);
var lastIndex = (r!=null) ? l.indexOf(r[0])+r[0].length : 0;
It is used here: http://www.pagecolumn.com/tool/regtest.htm