You can’t within the browser itself, as it uses RFP (resistfingerprinting) which is locked in stable (and you really don’t want to disable this anyway). RFP employs a feature called newwin to start all and any new windows at set sizes. RFP is all or nothing, you can’t turn parts of it on or off.
you can try Maximize All Windows (Minimalist Version) – Get this Extension for 🦊 Firefox (en-US) at your own risk - I’m not endorsing it, but it has been recommended by some arkenfox users