Firefox

Tab Snooze - allows you to close a tab and have it reappear at a chosen time later

Media URL Timestamper - automatically inserts the current timestamp of the YouTube/Twitch video you're watching and updates it in the history in case you accidentally close/navigate away from the page or go to a different time in the video

Feedbro - RSS reader with filtering capabilities

Redirector - auto-redirect specific URLs (for example, changing a YouTube Shorts url into a regular one, or changing Reddit links to always go to Old Reddit)

Undo Close Tab Button - adds a list of recently closed tabs to the tab context menu and allows you to restore them (including the tab's history in the back button) (max amount = browser.sessionstore.max_tabs_undo)

Violentmonkey - using userscripts that allow you to change things on websites.

• For Instagram - unmuting videos + setting their default volume
• For YouTube - disabling the subtitles/captions + disabling "autoplay next" for playlists + disabling autoplay of channels homepage video
  • a way to disable specific keyboard shortcuts (you need to manually add the code as a new script). I use /^(Key)?(End|I|O)|(Digit|Numpad)\d$/ instead of /^(?:Digit|Numpad)\d$/ (thanks to this post), to also disable the End/I/O keys in addition to the number keys. (note: this can also be used for other sites, but you might need to change stopPropagation to stopImmediatePropagation, for example on Patreon)
• Note that, at least for Violentmonkey, if the userscript doesn't have the "://" part of the url in the @match line then you need to add it in the userscript settings after installing the script (for example, if the @match line of the script only has *.youtube.com/* then put *://*.youtube.com/* in the "@match rules" line in the settings)

YouTube Comment Reader - allows you to search through the comments of a video (by clicking on the addon in the Extension menu and then clicking on the "YouTube Comment Reader" at the top or the "X Comments" at the bottom of the tooltip)

Page Shadow - allows you to use dark and light themes on sites that don't have the option to change it.

And if you're like me and you find that some YT videos feel too slow but 1.25x is too fast, then you can use Enhancer for YouTube's "Playback speed" feature to have smaller speed steps. Then you can hold ctrl and use the scrollwheel (while over the video) to change the video's speed by the amount you chose (I use 0.05 speed variation, mostly changing to 1.05x or 1.10x)