Discord Voice

Settings --> Voice & Video --> Noise Suppression. Set this to "Krisp".

uBlock origin custom filters

||external-preview.redd.it/*.gif?$image
(to block gifs on reddit)

the below are for the stackexchange family of sites
##.vertical-align-content.ot-wo-title.ot-bottom-left.otFloatingRoundedCorner
(some details about why it works))
###credential_picker_container
(hat tip)

SpellCheck

To get a list of all incorrectly spelled words:
cat file.tex | aspell list -t | sort | uniq
(hat tip)

A ffmpeg command

ffmpeg -ss 00:00:00 -to 00:04:12 -i bfbackup.mp3 -c copy output.mp3
This keeps the first 4 minutes and 12 seconds of bfbackup.mp3 and sends it to output.mp3.

Browser user-agent

(in firefox), go to about:config, then search for general.agent.override and create a new string.

Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/151.0.0.0 Safari/537.36

says that it's Chrome on Windows.
(for shitty websites that want a particular browser or operating system)