Do you need it only for web development? Try Responsive design mode in dev tools.

Changing the contents of the <title> tag, or showing an overlay in the corner of the document, should be easy for an extension or user script.

Detecting resize events is not complicated, I have an example in this page:

https://www.jeffersonscher.com/res/resolution.php

But I haven't searched to see what might be available. Did you try the Add-ons site?

Thank you, I tried the add-on site. With no luck. I just only like to have a short information of the display sizes when I resize the browser window, make it bigger and shorter- so I can check the design, but FLUIDLY, that I can see the size and the changing of the design in the breakpoints. I am NOT looking for a responsive layout checker.

Just a simple information of the domensions at the footer or somewhere else,..... I had that and got used to it. I like to have that back ;)

Hi Siseby, what was the extension you used before called?

Unfortunatly I cannot remember. But It was said, that it will not work in the update. It was shown in an add-on bar. https://addons.mozilla.org/de/firefox/addon/new-add-on-bar/

Yes, it's not possible to generate an Add-on bar in Firefox 57, so the information would need to go somewhere else, like a floating div in the lower corner of the page.

To find the old name, you could check the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then on the right side, check near the top for a "Show Legacy Extensions" link.

Thank you, it was the FIRESIZER ;) the dimension was shown in the add-on bar

As a temporary workaround to display the viewport dimensions, do you want to try a bookmarklet? You can install it from here:

https://www.jeffersonscher.com/res/sumomarklets.html#ViewportDims

Firefox will not run bookmarklets on sites with certain CSP rules, but otherwise it should work.

This is the script run by the bookmarklet:

function updateResolution(){
  if(!document.getElementById('vw_pnl')){
    document.body.insertAdjacentHTML('beforeend','<div id="vw_pnl" style="position:fixed;right:1px;bottom:1px;z-index:9999;color:#000;background-color:#ffa;font-size:12px;margin:0;padding:2px"></div>');
  }
  vwpnl=document.getElementById('vw_pnl');
  vwpnl.innerHTML='<b>CSS:</b> '+window.innerWidth+' x '+window.innerHeight; 
  if(window.devicePixelRatio){
    var wdpr=window.devicePixelRatio; 
    vwpnl.innerHTML+=' <b>Device:</b>'+Math.round(window.innerWidth * wdpr)+' x '+Math.round(window.innerHeight * wdpr);
  }
  vwpnl.innerHTML+=' <button onclick="document.getElementById(\'vw_pnl\').style.display=\'none\';" style="font-size:12px;line-height:13px">x</button>';
}
var resizeTimeout; 
function resizeThrottler(){
  if(!resizeTimeout){
    resizeTimeout=setTimeout(function(){resizeTimeout=null;updateResolution();},200);
  }
}
window.addEventListener('resize', resizeThrottler, false); 
var vwpnl=document.getElementById('vw_pnl');
if(vwpnl){
  vwpnl.style.display='block';
}else{
  updateResolution();
}

That is great! Thank you so much. It will help me a lot. Thank you again for your time and efforts! Probably there will be updates in the add-ons sooner or later. Until then: perfect!