Limited to a selection of pre-approved extensions (Ublock Origin is included in said list); however, you can get around this by using Firefox Nightly and enabling developer settings, which allows you to add a custom list of extensions, which can include any extension for desktop Firefox, including things like SponsorBlock. (Credits to https://lemmy.world/u/Pr0phet )
You can use the Chromecast with Revanced at least, but you'll need to use the TV code found in the settings of the Chromecast's YouTube app and you will still get ads