Some ways to try to revert it (some have already been mentioned)

👉 Go to this page and check if you’re able to opt out of the “Redesigned Watch Page” (source)

👉 Install a userscript addon like Violentmonkey, and then use a script like Classic Youtube Layout (which is based on this script which was outdated in regards to the related videos sidebar)

• If you get an error about the @match line of the script then click on allow edits and change it from *.youtube.com/* to *://*.youtube.com/*, or install the script as it is and then put *://*.youtube.com/* in the “@match rules” line in the settings tab of the script
• There’s also this script that you would need to add its code manually to the userscript addon by clicking on the new script icon (see the author’s post for more details)

👉 Use uBlock Origin filters like these or these

• Note that youtube.com##+js(set, yt.config_.EXPERIMENT_FLAGS.kevlar_watch_grid, false) should be enough to change the UI back for the most part

👉 Use this userscript and Stylus style

👉 The addon Enhancer for YouTube (Firefox / Chrome) also reverts the UI, at least partially

👉 (might only work some times) Use the addon CustomTube - Firefox / Chrome (the default UI is from 2017 but you can choose 2021 in the settings. It’s not exactly like the one from 2023 but it’s close enough)

• note that it removes some buttons like Join by default so go over the settings if relevant

If you also want to change the number of videos per row in the subscription page and channels’ videos page, then you can add this uBlock Origin code to the “My filters” tab (at the end click “Apply changes”, or Ctrl+S) (this is for 6 items per row but you can change the number in the first 2 line as needed) (source 1 / source 2)

youtube.com##ytd-rich-grid-renderer:style(--ytd-rich-grid-items-per-row: 6 !important;)
youtube.com##ytd-rich-grid-renderer:style(--ytd-rich-grid-posts-per-row: 6 !important;)
youtube.com##ytd-two-column-browse-results-renderer.grid-6-columns:style(width: 100% !important;)
youtube.com##ytd-rich-grid-row, #contents.ytd-rich-grid-row:style(display:contents !important;)
youtube.com##ytd-two-column-browse-results-renderer.grid:not(.grid-disabled):style(max-width: 100% !important;)

Sorry, it was unclear in my comment. By “it’s actually part of the Mozilla Corporation” I was referring to Firefox, not Mozilla Foundation

for anybody that wants to disable it, go to the settings and search for “Allow websites to perform privacy-preserving ad measurement”

(or through the dom.private-attribution.submission.enabled flag in about:config)

https://support.mozilla.org/en-US/kb/privacy-preserving-attribution

Just adding that as I understand this, donations to the Mozilla Foundation cannot go towards Firefox, because it’s [edit: Firefox is] actually part of the Mozilla Corporation. To help with funding Firefox people can consider purchasing the Corporation’s other products (VPN/Relay/Monitor), or purchasing merch.

See more here on the AMA on Reddit, and this thread