I would prefer adding a more generic section explaining how to pass options to the drivers, similar to the existing Passing Arguments to ChromeDriver section. WDYT?

No, sorry, I have too little overview.

• Some headings explicitly mention Chrome (e.g. Passing Arguments to ChromeDriver
• Some headings mention a topic (but no platform), but the text only addresses Chrome Accepting Self-signed SSL Certificates, see Adding self-signed certificates for Firefox #456

So while I do agree with what I think is your bottom line (namely that README is a bit unorganized right now :-) I can't guess in which direction you want to evolve it ;-)
Ultimately, the page is getting to long anyway, and should be separated into a dedicated (sub)website. But since this would probably be a larger undertaking, the best guess for now is probably: Merge this as is, and do the larger reorganization later...

Maybe should we just move these docs in symfony/symfony-docs? Panther reached 1.0 anyway, so it's probably the good time. WDYT @symfony/team-symfony-docs?