YouTube made a change recently but Firefox is still taking their time to review version 2,3.5 which fixes this.
If you want to update right away, you can switch to the beta version.
You can install the beta on Firefox by clicking "FirefoxSignedInstaller" on https://github.com/ajayyy/DeArrow/releases/tag/2.3.5
You'll have to disable the main version. You can copy over your setting by first going to DeArrow -> Options -> Backup/Restore and saving the settings file