Try the Firefox version from the official Mozilla server if you currently use a version from the repositories of your Linux distribution.

This issue can be caused by corrupted cookies or blocked cookies.

• check the permissions for the domain in the currently selected tab in "Tools -> Page Info -> Permissions"

Details like websites remembering you is stored in a cookie.

You can use these steps to make a website recognize and remember you.

• create a cookie allow exception with the proper protocol (https:// or http://) to make a website remember you

You can check that you aren't clearing important cookies.

• using "Delete cookies and site data when Firefox is closed" keeps cookies with an allow exception
• using "Clear history when Firefox closes" to clear cookies clears all cookies including cookies with an allow exception you may want to keep
• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data

• Options/Preferences -> Privacy & Security
  Cookies and Site Data: "Manage Exceptions/Permissions"

I have the "official" download. I asked for a simple fix, I guess there isn't any and there isn't going to be.

YouTube stores some settings in cookies and some other settings in local storage.

You are using Firefox from the Ubuntu repositories and Ubuntu has the habit to brand their version with all kinds of tweaks, so it is quite possible that some affect the volume setting on sites like YouTube.

• Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:89.0) Gecko/20100101 Firefox/89.0

See also these prefs on the about:config page.

• media.default_volume 1.0
• media.volume_scale 1.0