accessibility.force_disabled I build with ac_add_options --disable-accessibility, so not applicable here.
app.shield.optoutstudies.enabled is already set to false.
beacon.enabled is still true since it seems to be a web standard, and I don't want to break anything that may use it. However if it is bad let me know.
browser.laterrun.enabled is already set to false.
browser.shell.checkDefaultBrowser I want to keep at true because I don't want to confuse new users if they want to set as default and are used to the prompt.
browser.slowStartup.notificationDisabled has no mention in the Firefox source code, so would be useless to set.
browser.uitour.enabled again seems like something that could confuse new users if disabled, so I'd rather keep it on.
default-browser-agent.enabled I build with ac_add_options --disable-default-browser-agent, so not applicable here.
extensions.update.* Automatic extension updates are fine, some just need to update like uBlock Origin with everything YouTube is trying to do.
I'd rather not mess with rendering stuffs and anger people if the defaults work fine for most.