#floating commenting box

Step 1: Install Tampermonkey web extension on your browser

(You can skip this part if you’re used to install browser extensions)

On Chrome: Type chrome://extensions in the address bar. It normally shows your installed extensions.

Click on the menu button in the top left corner of this page. At the bottom there's an button linking to the Chrome Web Store. Click it.

In the web store search bar, type “tampermonkey” and select the first result.

You should land on a page looking like this:

Click on "add to chrome" and follow the installation procedure (you'll have to grant a few permissions for the process to fully complete).

On Firefox: Type about:addons in the address bar. You land on a page listing your already installed addons (if you have any).

In the search bar at the top, type "tampermonkey" and enter. It should bring you to another page looking more or less like this:

Click on the first result in the list (the black logo with the white two circles) and follow the installation procedure. You'll have to grant a few permissions when prompted for it.

Step 2: Install AO3 script on tampermonkey

Click on the newly installed tampermonkey icon (depending on your browser parameters, it can directly show in your toolbar, or be hidden, for instance you have to click on the little jigsaw puzzle icon in Chrome's main bar and select tampermonkey for it to show in the tool bar).

Click on Tampermonkey icon. You open a menu looking like this (my interface's in French, but yours should at least feature the same lines/symbols):

Click on "add a new script" (the '+' symbol).

You land on a new page consisting of a code editor. Delete ALL the default code already in the editor.

Open a new tab. Scroll down this page and click inside the "RAW paste data" box. Ctrl+A. Ctrl+C.

Paste the code you just copied in the aforementioned editor tab. Ctrl+S.Normally the page turns into this, which means that the script has been installed:

Open a new tab and go on any AO3 fic (or refresh one that is already open).

The floaty box button should appear along with the usual buttons on top of the fic page.

De-activating/uninstalling the script

If you ever get tired of it, you have four different ways to not use this script anymore.

You can deactivate it in Tampermonkey: go an AO3 page (you need to be on a page where the script is active to see it in the menu), click on the tampermonkey icon and then the script name line in the menu to de-activate. You will be able to re-activate it anytime you want. This way you can also keep using the other scripts you may be running with Tampermonkey.

You can delete the script from Tampermonkey: Click on the "dashboard" line (the penultimate one) in Tampermonkey's menu. A list of all the script you have installed shows. Find the line about the script you want to delete. Click on the little trashcan icon at the end of the line.

You can deactivate Tampermonkey itself. Click on the first line in Tampermonkey’s menu. This will de-activate the extension, so EVERY script you've installed on the extension will get inactive as a result. You can re-activate the extension anytime you want.