Can I really, in Firefox? This seems like something that's impossible to provide via the Web Extension API.
Maybe hasn't been done, but I agree with some of the other comments around here that if this were truly important, someone would've done it. Or at least if a browser is going to support it, it should only be enabled via some hidden dev setting.
That's basically a variant of the efficient market hypothesis for open source software. I highly doubt it's accurate, for various reasons.
Things that people retrospectively consider absolutely essential and a no-brainer to prioritize get shipped years after the initial release of some software all the time.