Our screens have gotten wider. Our browsers have also gotten wider. But our tabs remain a thin, barely visible line drawn across the top that become two letters when we have more than 10 tabs open? This is 2021. You need side tabs.

1. Download and install https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/
2. Enable userChrome.css https://winaero.com/enable-loading-userchrome-css-usercontent-css-firefox/
3. Open about:profiles and click Open Folder next to the Root Directory.
4. Create a folder named chrome
5. Create a file named userChrome.css inside that folder.
6. Paste the following into it and save

.tabbrowser-tab:not([selected]) {
    display: none !important;
}

.tabbrowser-tab[visuallyselected="true"] {
	max-width: none !important;
}

Enjoy!