May 11, 2026: I installed "Viewport" in Firefox 150.0.2 and like many people in the reviews, my first experience was that nothing happens when resizing a Firefox window. I got it working, so read on for the full story.

First, I opened a new browser window to test the add-on. I visited a search engine site. Resizing the browser window was working, but there was nothing being displayed on the screen as expected from the add-on.

I have the Extensions button in my toolbar, and from its drop-down menu I could see that Viewport had "Can't read and change data on this site" next to it. This is in contrary to the pop-up when installing the add-on, which said that it needed the permission "Access your data for all web sites".

How I fixed it: I needed to into Manage Extensions (about:addons), Viewport, Details and change "Run on sites with restrictions" to "Allow". (Even then, it will not work on addons.mozilla.org, you will need to open a different site in order to test the add-on after installing it.)

Back in my test window with the search engine page, resizing the window now displayed dimensions in the top-right corner of the viewport (i.e. the top right of the web page).

I then tried changing that setting back to "Don't Allow", and the add-on kept working in my tests. I also uninstalled the add-on and reinstalled it, making no further changes to its settings, and it still kept working!

The only things that I did not try were a full exit and restart of Firefox, and installing the add-on again in a new user profile.

As the last release of the add-on was in April 2024, it's almost certainly a change in Firefox that has resulted in this strange experience.

Further testing on random sites showed that the add-on works well with Firefox in "Light" mode (in the "Web site appearance" Firefox setting). In "Dark" mode, and/or on sites with their own "Dark Mode" setting that I had enabled, the text from the Viewport add-on was not always easy to read (and sometimes impossible to read).

Overall, 4/5 stars because the add-on works for me after a little fiddling, and the add-on does the one thing that it says it will do. If you want no more features than this one simple thing, the add-on may be suitable for you! I'm leaving it installed, but disabled, and will only enable it again in Firefox via Manage Extensions (about:addons) when I need to check the viewport size, as I don't want to be seeing this all the time.

I use this and think it works well. I find I must disable it when I want to interact with content on certain pages. Even though the add-on is not active (I'm not resizing the window), it seems the overlay element gets loaded and I either have to manually display:none the invisible element or disable to add-on.